Quarantine does strange things to ya, folks. I only ever watched the first few seasons, years ago. One thing I recall is that I always wondered what would change if Stiles had to actually cut off Derek's arm back in S1, never found fic exploring that, and so last April out of the blue I wrote fic exploring that.

Warnings: Graphic violence/gore, panic attacks.

Stiles vomits, after.

It is after, though. That's important. Very, very important actually, because – because werewolf healing might be incredible, but Derek's already going to have to contend with an amateur amputation, having vomit in the open wound on top of that just kind of seems like too much to ask. It's also important because Stiles went down a fortnight-long survival story spiral back in eighth grade, mostly natural disasters but also stuff like Hatchet and that true story about the sports team who ended up having to eat their dead friends to stay alive, and he'd wondered, as one does, if he'd ever be capable of that. Sort of in that half-hearted way, where he obviously didn't ever actually want to find out if he'd be able to eat dead Scott's buttcheek to keep himself alive long enough to make it back to his dad, but also it felt important to hope he'd be capable of whatever he had to do, because his mom had just died and his dad felt dead and he was in a pretty much hideous mental space but he didn't want to give up. And now he knows he won't, not till after. Not that he's going to need to eat Derek's arm, not that this is about cannibalism at all, that was just an example, but Stiles knows now that he is mentally and physically capable of cutting someone's arm off to save their life. Without vomiting in the middle.

He vomits after though. Just – straight onto the floor. He figures it'll be fine, because after all this is the room where Deaton does his surgeries so it's designed to be easy to clean. Also, there's still all Derek's gross black vomit everywhere, and blood (lots of that, lots of that) and little bits of – of bone dust, muscle gristle, whatever the fuck those specks are. And, also, Derek's arm. On the floor. So a little normal vomit that's mostly bile anyway, doesn't seem like much more of a hassle. Probably barely noticeable.

Derek passed out, somewhere in the middle of the process. He'd been screaming, before that: horrible, wrenching screams digging their way out between gritted teeth, his other hand clutching so hard on the table it crunched around his fingers, so so hard, holding himself still for Stiles to chop up –

He'd yelled at Stiles a lot, before they really got started. Back when Stiles was still trying to stall for Scott, back when he was insisting he was going to pass out, when he tried the first time and only managed to make a shallow cut before flinching away so hard he almost smacked the saw into the table. Derek had said some very, very rude things. He'd insisted Stiles look.

Which, he knows wasn't actually sadism, or some sadistic variation of masochism, just necessary so he didn't cut off the wrong appendage or something, but he's pretty sure he'll never forgive Derek for that regardless. For this whole afternoon, frankly.

After he pukes all over the floor, Stiles claws himself back to standing. He looks again, makes himself look. It's – it's probably the worst thing he's ever seen, he doesn't want to describe it, he, he has to puke again but there's nothing to come out.

Stiles is pretty sure that there are a lot of things you're supposed to do for an amputation, none of which he is capable of doing. He's glad at least that for their second try, Derek sat down in a chair, because he hates to think of what would've happened when he passed out if he'd just been bent over the table like before. As it was he went limp and slid a little, and Stiles had to yank the saw back before he cut a T-shape into him or something. The cut's probably a little uneven now because of it. He can't really tell because of all the blood.

Stiles takes off his button-up, wads it up against the stump of Derek's arm, tries to figure out how to tie it in place without letting go. Everything's too far away though, so he leaves the shirt, already soaking through, and rushes to the cabinets to try and find some actual bandages or something. The first one he tries is locked, and he's crying, fuck, fuck fuck fuck he just cut off Derek's arm GOD –

He's not breathing. He realizes that too late, too far into the panic attack for it to come back as anything but hyperventilating. And it's not like usual, where he tries to convince himself nothing's wrong, nothing's going to hurt him, he can't make himself count in and out because that takes time and in that time Derek is bleeding out, and Stiles isn't helping him, he could die, one of the Argents has killed him and wants to kill Scott and he's at their house right now and Stiles will probably be next and Derek has only one arm and, and, Stiles wants his dad.

Stiles passes out. This one, he's less proud of, because he should've waited until after. Until the bandage was on. He's aware, even as he passes out, that he hasn't finished the job, and that's probably worse even than if he hadn't started.

If he were starving on a mountain, he'd probably successfully eat Scott's buttcheek, but then puke it out before digesting it. All the horror, none of the lifesaving.

-xxx-

He wakes up to Scotty's face. It's horrified – not like he's scared, but just. Horror. An unholy mix of guilt, pity, disgust, and yeah, some terror as well. He's shaking Stiles, a little too rough.

"Stiles, please, please wake up," he's saying. "I'm sorry, Stiles I'm so sorry."

"Derek's dead," Stiles croaks, before he even remembers what's going on. Then he does, and lurches up, head spinning, "-oh, shit, Derek's –"

"He's breathing," Scott says, puts a hand on his chest to keep him down. "I, I put a bandage on. Hey, slowly."

Scott helps Stiles up, nice and slow. Pulls him up to his feet, supports him on his shoulder when Stiles is more than a little limp, finally leans him gently against the counter. Stiles stares at the top of Derek's head, slumped against the surface of the operating table. He thinks how Scott just said Derek was breathing, not okay.

Yeah, obviously.

Scott is pale and keeps looking around the room. "I – You… Stiles, you…"

"I cut his arm off," Stiles says dully. The room feels very still. "He made me do it."

"Why would he make you –"

"So he wouldn't die, Scott," Stiles says. It's the perfect moment for sarcasm, wit of some kind, but he's got nothing. "He was dying."

Scott looks down, puts a hand in his pocket. Stiles stares at Derek some more.

"What do we do now?" Scott asks.

Stiles doesn't want to think.

"We have to get out of here," he says after a moment, because the choices are not thinking and letting them all get arrested by his dad for mutilating the guy who might have murdered his own sister and then all getting murdered in their cells by the Argents or the Alpha, or thinking and somehow getting them all out of this alive.

It's too late for in one piece.

"We've got to – to take him somewhere. Your house," he tells Scott. Scott's face scrunches up in distaste, which, yeah, Stiles gets, but –

"I can't bring him home! My mom might see!"

"Yeah, and if we take him to my house he'll get arrested and then he'll get murdered successfully next time! At least if your mom sees him she'll probably know what to do for –" Stiles waves an arm at all of Derek, rather than saying anything specific.

"Maybe we should just let him get murdered," Scott mutters resentfully. Stiles understands, he fully does, but.

"Scott, if you let Derek die after I cut off his arm to keep him alive, I'm going to murder you," he says firmly. He means it. It's obvious how much he means it, his voice is so cold he almost doesn't recognize it as his own. Scott quails, and finally agrees.

"So. So you take him home, and I'll, I'll clean this up," Stiles says. He reaches into his pocket, pulls out his keys, hands them over. "Come back and get me when you're done."

"Maybe I should do this?" Scott looks adorable when he's worried about him. It always makes Stiles feel all warm and fuzzy inside, except for right now, when all he feels is this kind of low buzz.

"There's no way I can carry him anywhere," Stiles points out, very logically.

Scott hesitates, then swallows hard, and says, "Okay. Okay. I'll be back soon, okay?"

"Yeah. Go, I'll be here. I'll be fine," Stiles lies.

-xxx-

There's a mop in a back closet, and a lot of cleaning materials. Stiles washes his hands first, because he doesn't want to get blood on the inside of the big rubber cleaning gloves. Then he tries to wash his arms, as much as possible, and his face, and realizes he'd definitely had his mouth open because he'd been screaming too, and blood had been going everywhere, and –

He swishes and spits. Doesn't see any blood come out, but maybe he just puked it up earlier. Hopefully he's not going to contract some horrible disease or get wolfsbane poisoning himself or become a werewolf if he really did swallow Derek's blood. He'll have to ask Derek if that's a risk later.

For now, he gets as much of himself washed in the little sink as he can, then takes the mop back into the room, and cleaning spray, and a roll of paper towels, and gets to work.

He starts at the top, standing on the chair and scrubbing at all the cabinets. Standing on the operating table to wipe the ceiling down too, getting the countertops and the table itself and the chair. Then the bone saw, even though picking it up again fills him with a sudden, intense urge to smash it to pieces on the floor. Stiles is very thorough, enough that he winds up choking a little from all the fumes, feels kind of dizzy by the time he's done.

Then again, maybe that's just the shock. He's gotta be in shock.

He leaves the room. Breathes heavily over a trash can for a while.

When Stiles comes back in, he's got a big black trash bag in his hands. He uses it to pick up Derek's arm. Picks it up with the gloves still on his hands, arm in the trash bag like he's picking up a dog poop. Most of an arm is bigger than a dog poop, though, and it's hard to manage, he has to adjust and use both hands and then kind of roll the trash bag down over them, and at some point in the process he can tell he's holding onto Derek's hand. He can feel his fingers through the bag.

He puts his bloody shirt in too, ties off the bag, then takes off his gloves and puts those into the little trash can in the room. Then Stiles sits down on the floor and tries to breathe. Steadily. In four, hold seven, out eight. In four, hold seven, out eight.

(Normally he'd look for five things he could see, but. He doesn't want to see anything in here. So just breathing. He can do that.)

In.

Hold.

Out.

Scott.

Scott.

"Hey, buddy," Scott says, crouched in front of him. "Hey. I can get the rest of this, okay?"

"Hi, Scott," Stiles says. "How's Derek?"

"He's sleeping in my bed. How are you?"

"Oh, I'm peachy," Stiles says. "Can I have a hug?"

Scott gives the best hugs. Just tight enough, just warm enough, just long enough that time forgets about you if you close your eyes. They're second only to Dad Hugs, which are magical enough to completely deserve the capital letters and also probably cure cancer.

One thing they both do, one thing all truly excellent huggers do, is know just when to pull away. They both have the gift of knowing exactly when you start thinking maybe you should let go, and relaxing right before you have to. It's a really nice touch, to the whole hugging experience.

This time the hug is probably five minutes long. They just hold on, tuck their heads against one another, and at maybe the four-minute marker Stiles realizes he's smelling Scott's hair and feeling somehow very comforted by the smell. Almost enough that he feels bad for teasing Scott about werewolf noses (but not quite).

When they let go, he tells Scott he'll call him tomorrow, and thanks for mopping up all the vomit and stuff, and to take out the trash with the gloves and all the paper towels he'd used, and then he picks up the plastic bag with Derek's arm in it, gets in his car, and drives way out into the preserve.

He knows there's not a great track record lately of keeping body parts hidden in this forest, what with both halves of Laura Hale's body being discovered pretty quickly, and a crazed werewolf running around here all the time, and hunters too, but he also is a hundred percent certain that if he just threw it into a dumpster somewhere, whoever found it would report it to his dad, and what with the shirt in there too they would find Stiles immediately. At least this way, most of the people who will find it probably won't be racing to call the cops.

He's still not sure where to put it. And he doesn't really want to hold it for too long. And he doesn't have a shovel. So eventually, he just parks on the side of the road, holds the bag between the tips of his fingers, and walks just past the treeline, using his phone as a flashlight. He was going to just drop it in some leaves and go, but then he sees a hollow log, so he shoves it in there instead. Stuffs some leaves in after it, and walks back to his car, simple as that.

Stiles drives home and it's terrifying, somehow, to walk into an empty house. He knew he would, it's the reason he was so scared the whole time he was cleaning up his crime scene and dumping Derek's arm, because he knows his dad is working late tonight, but somehow he'd still expected to see him at home.

It's lucky he isn't there, because once Stiles goes upstairs and looks in a mirror it's apparent he did not get all the blood off of himself. His clothes are splattered with blood, so he takes them off and walks naked to the washing machine. Runs them on the rinse cycle with cold water, and while that's going he trudges back upstairs, still naked, to pull his sheets off the bed. He brings those down, crumples them up on the floor, knowing that if his dad sees sheets in the laundry room he won't come any closer. Once the rinse cycle is finished, Stiles pours in probably too much bleach, and laundry soap, and starts the machine.

He spends two hours in the shower.

Brushes his teeth for five minutes straight. Almost has a panic attack when he spits and sees red, before he realizes he's just made his gums start bleeding.

Finally steps out of the bathroom, in a rush of hot, humid air. Puts on pajamas, a robe, slippers. The house is very quiet.

Stiles puts his clothes in the dryer, then his sheets in the washer, then goes to the kitchen and looks in the fridge for something to eat. There's still some pasta from two nights ago, so he microwaves it and eats straight out of the container until it's gone. He goes upstairs, into his room, lays down on his mattress. Closes his eyes.

(He'd had to push. It hadn't gone in easy, not without any effort even in the beginning, but once he hit bone he'd had to push down, Derek's eyes had been wide open and bright blue, he'd screamed and then passed out and Stiles had to stop. Had to start again, to put the saw back in the same spot and turn it on again and push–)

Stiles takes his pillow and blanket to the couch and watches HGTV until his dad gets home.

-xxx-

Scott calls him at eleven in the morning. Stiles wakes up to the sound of his ringtone, tries to reach for it, and falls off the couch. He groans into the carpet, doesn't move.

His dad drops his phone on him. It bounces off his chest and falls to the floor next to him, still ringing.

"You're welcome," Dad says.

"Mmph," Stiles tells him, then fumbles to hit the answer button with his eyes still closed. "Hmmf?"

"Stiles? Are you up?" Scott's whispering. "Can you come over? He's awake."

Stiles opens his eyes.

"Yeah," he says, heart sinking right out of his chest. "Yeah, I'll be there soon."

After he hangs up, he goes into the kitchen. His dad is setting the table. The air smells warm and sweet. There're waffles already sitting on the table, and a second later the microwave beeps and Dad gets out the syrup bottle. Stiles' throat closes up.

"Dad," he says, tightly. Has to bite his lip and swallow before speaking. "You know syrup is a contraband."

"I get a cheat day," Dad says. "One a month. You promised."

"Yeah, but a whole bottle of syrup? That's like crack for you. We can't just leave it here, you'll put it on everything."

"Not if we use it all in this one meal," Dad says like he's being clever, but when he sits down and pours syrup over his waffles, it's a normal amount. He adds butter too, quickly before Stiles can protest.

Stiles is still just standing there. His mom used to make waffles every weekend. After she died, Dad had tried and tried to perfect the recipe she'd used, since she'd never bothered to write it down. Eventually, he'd figured out something that tasted close enough to ache, and then he'd put the waffle-maker in the back of the shelf for three years.

They eat waffles on her birthday now. Only ever then.

(But Dad had come home after midnight to find Stiles huddled up in a tiny ball on the couch, watching brainless TV with wide dry eyes, and breathing in a deliberate pattern. He'd locked the door behind him, took off his coat and his gun and shoes, and then sat down next to him without a word. Had wrapped his arms around Stiles and held him, silently, for hours until he finally fell asleep.)

"Hey," Dad says. He waits until Stiles meets his eyes before continuing, with an awkward little smile: "C'mon, kid. Don't let 'em get cold."

Another day, another breakfast, and Stiles would leave. He'd hate to do it, but Scott and Derek's situation is still a literal life-or-death problem, so he'd leave his dad with a stupid excuse and feel miserable about it all day and do it again the next time it happened, no hesitation. But these are Mom's waffles, half a year out of season.

Stiles plops down into a seat and eats half the stack.

Midway through, Scott texts him, asking where he is. Stiles puts his phone on silent under the table, with a vicious little thrill of retribution, and takes his time eating.

-xxx-

Derek's passed out in Scott's bed, pale as death. Scott is glaring, but frankly Stiles still doesn't care. He has the best dad in the world and sometimes that needs to be acknowledged. Besides, Melissa isn't even home since she's covering a shift for a coworker so it's not like there's anything to worry about.

Other than Derek maybe dying or something, but apparently he woke up earlier long enough to eat a granola bar then pass out again, so. He'll probably be just fine.

Stiles expresses this opinion as nonchalantly as he is able, just to test out how nonchalantly that is. He thinks it sounds impressively like he doesn't care at all whether Derek lives or dies, but Scott's eyebrows knit in concern and Derek wakes up seconds later like he can smell the lie even in his sleep or something. So maybe that needs some work.

He makes a horrible little noise, like he's going to choke. Then it turns into a deep, terrifying growl that has no right coming out of a human throat, and right when Stiles is getting afraid that he's going to be murdered in some feral injured wolf rage, it trails off into a whimper as Derek tries and fails to sit up.

"Hey, take it easy, big guy," Stiles says. He's sitting at the foot of the bed, and when Derek looks at him, he has to turn his gaze to the floor. Scott obviously sleeping-bagged it last night, and even though he's since put it away, there's a conspicuous clean spot amongst the mess on the floor. It's kind of cool, like a negative space art piece or something.

Derek breathes heavily for a few minutes in silence. Then he clears his throat.

"Water!" Scott jumps up. "I'm gonna get you a water bottle, okay dude?"

He bolts before Derek can say anything, or Stiles could offer to go instead. Without him in the room, the silence gets even worse. Stiles can feel the bed moving slightly, as Derek tries to scoot into a sitting position, more carefully this time. He should probably offer to help or something, but he can't make himself move.

Derek keeps making bitten-off little noises.

Stiles glances at him out of the corner of his eyes, once they stop, and it seems like he's done for the moment. He's sitting mostly upright, staring down at the space where his left arm used to be and there's currently just a lot of messily wrapped bandages.

He looks up, right before Stiles was about to look away again, and then it's impossible to look away.

Derek swallows.

"Thank you, Stiles," he says, very quietly. And Stiles doesn't know what he expected, but it wasn't this, sincere words and solemn sad eyes and the memory of Derek screaming under him, at him. He went home last night and pictured Derek dying, over and over and over, but somehow it's only right now that it occurs to him this isn't anything new for Derek. He'd thought the guy would hate him for cutting his arm off, even if he was the one who forced Stiles to do it, but Derek's face looking at his arm was so blank.

It was the same face he makes when he talks about the Argents, whenever any mention is made of the fire. It looked like Stiles felt, last night, when he made the choice to do something terrible so no one would die. When he forced himself to think, to clean up the crime scene and dispose of the evidence, and clean himself and get through it because nothing else was even an option so he just shut everything else down and did what he had to do.

Stiles had been thinking Derek was this scary, weird guy, probably not evil but definitely kind of a jerk and not someone he really would want to spend any time with if he weren't insanely curious about all things werewolf. He'd been more than willing to cut him loose if it came down to any sort of choice, because he and Scott were a team and keeping them safe mattered way more than Derek. He's a big tough werewolf, he should be able to take care of himself.

He can't.

Derek, Stiles understands abruptly and with a horrible sort of lurching feeling deep in his chest, is still in emergency mode. In some form of shock, leftover from the fire years ago, and everything that's happened in Beacon Hills has probably just made it worse, bit by bit. It's something Stiles probably would have figured out eventually, given enough time, but right now the understanding hits not only suddenly but also with weight.

He cut Derek's arm off last night.

That was a point of no return, in so many ways. Stiles stares silently at Derek, for way too long, and Derek just looks quietly back, has just thanked Stiles for hurting him because at least it's not dying. And Stiles realizes he can never let him die, now. He's not sure he'll ever be able to see Derek hurt at all without puking again, but he's hurt all the time anyway, at the very least he can't let him die.

Scott bursts back into the room with several bottles of Gatorade instead of water, a handful of granola bars, and an entire bunch of bananas stuffed under one arm. He's already talking about vitamins and electrolytes and maybe changing the bandage or something, but Stiles and Derek just keep on staring at each other for several seconds more.

Stiles feels shaky when he finally turns away.

Derek accepts the food and Gatorade, and it was a good idea to bring several, probably, because he downs almost the entire first bottle before surfacing for air. He eats quietly, watching Scott as he talks circles around his visit to the Argents' house, never quite apologizing even though he is obviously spilling over with guilt, until finally Scott swallows hard and admits he got the bullet. He points, and there it is, just sitting on his desk.

"Thanks," Derek says, which much less sincerity this time. Really more aggressive than most death threats rather than anything approaching gratitude. Scott flinches a mile high, and Stiles feels a moment of sheer rage that he did all that (he did something monstrous) for nothing – but Scott hangs his head and maybe it would have been too late anyway, and he'd tried, and he'd bandaged Derek up and carried him home and come back to mop up all the blood – and Stiles can't do it. He's incredible at holding a grudge against anyone but the people he loves, and he loves Scott.

"Hey, do you think you'll be able to grow it back?" he asks Derek. This question shouldn't work at all to lift the tension, and Scott gives him a clearly horrified glance, but the actual newly-minted one-arm wonder seems weirdly not offended.

"Maybe if I still had the arm," he says.

"Oof, yeah, that's um, not an option," Stiles grimaces. "I, uh, dumped it in the woods? Probably raccoon-food by n–"

"Stiles, shut up!" Scott hisses. Derek looks distinctly ill. Stiles shuts up. For a few seconds.

"Need a trashcan, there, buddy?" he asks Derek.

The werewolf whips his head up, glares at Stiles, then kicks him off the bed. Stiles goes down with a yelp, whacking his elbow on something hard, and curls up around it in pain. As the hurt subsides, he hears Derek above him telling Scott, in a very serious voice, "Now do you understand how dangerous the Argents are?"

"Allison's not like that–"

"Oh, Jesus," Stiles moans. When they both try to keep talking, he does it again louder, sitting up. "JESUS, would you both stop? Scotty's in love, Derek doesn't want everyone to get murdered, we all get it. Can we just, just work as a team, without all the macho posturing? Yeah?"

They both look at him with betrayed expressions. Well, Scott looks betrayed, and Derek looks some combination of disapproving, approving, thoughtful, and just generally scathing. Stiles… isn't really sure what to make of that, but at least the feeling seems mutual, because after a moment, Derek's eyebrows clearly signal confusion. Presumably at the part where Stiles is implying they work together with him.

"We need to figure out a plan," Stiles says, swallowing down the sick in his stomach, "okay, all together for once, and then actually follow it, and then maybe none of us are gonna die, which is frankly my one and only priority right now. Which is a travesty because I should be working on my Lit. essay if I ever want to come anywhere close to Lydia Martin, goddess that she is, gradewise and hopefully also in other ways, but instead I'm spending my Friday nights dumping body parts in the woods and I just don't want any more maiming of the good guys, okay? It should not be too much to ask."

Scott obviously can't miss how he's talking about all three of them as a team. As a pack, maybe, whatever the hell that involves, and he just as obviously doesn't approve but Stiles meets his eyes and glares him down. And maybe he remembers last night when Stiles told him not to let Derek die, maybe even if he doesn't feel it deep deep inside the most vicious part of him like Stiles does, maybe he at least understands that Stiles does, that letting Derek die is no longer an option for any of them. And that means working with him, keeping an eye on him, because he can't keep out of trouble on his own, he's been treading water way too long already. It's so obvious now.

"Fine," Scott says. "Okay, but do we have to do that right this second?"

Derek still looks confused.

"Yes?" he says, looking back and forth between Scott and Stiles. "We should –"

"God, no," Stiles says over him. "We should obviously just watch movies all day right now. We have Derek trapped, now's the time to force pop culture down his throat. I bet he hasn't even seen Star Wars."

"Wh- no, we should figure something ou- of course I've seen Star Wars," Derek says, offended. "I'm not a caveman."

"Well, I guess that makes two of us," Stiles gives Scott some significant side-eye.

"Would you stop already," Scott moans, but the fact that even Derek is more pop-culture savvy than he clearly makes an impression, and he mumbles in clear defeat, "I'll go make popcorn."

Stiles cheers, scrambles to open the box set he bought Scott for Christmas – last year – and tells Derek, still obviously completely confused, "Just go with it, all right? You're all – convalescing. Just convalesce."

"We can't ignore the Argents, Stiles," Derek bites out.

"Okay, fine, sure, but they probably think you're dead right now, so we can at least take the day after I cut off your arm off. All right?"

Derek looks guilty, at that. At making Stiles cut his arm off. Which, yes he should be, but also he's the one who lost an arm, so the guilt is more than a little unsettling.

Jesus, Stiles thinks again. Remembers Derek threatening him, insulting him, snarling viciously, clutching the table till it bent (shit, he forgot about that, nothing for it now), anything at all not to die.

"All right," Derek grumbles.

"Yeah, you are!" Stiles says, and finger guns. Derek looks confused for a second, then furious, then snatches up the one pillow he isn't using behind his back and flings it with great force at Stiles' head. It actually knocks him down, which is way more powerful than a pillow projectile ought to be.

This will work, he thinks, laying under the pillow. He'll – somehow, he'll make this work. Get them all through it.

Just save the breakdown for after.

So in this verse I imagine they would get the parents involved way quicker, because they just wouldn't be able to hide Derek and keep him safe long enough. They'd be a team from here on out, though, largely because of Stiles, but eventually Derek and Scott bond more on their own merits. Because I like the dynamic, I picture Derek moving in with the Stilinskis and getting therapy. (Stiles needs it too.) He'll still probably become alpha, still probably turn at least some of the pack, but probably not the same path there. No way the group'd blame him in that school attack episode, for instance, and other stuff like fighting would be affected obviously.

He would not grow his arm back, but it would heal over unnaturally quickly. This would be the main reason they'd end up having to let the Sheriff in on the truth as soon as he lays eyes on Derek, because he just arrested the guy like a week ago and he did not have an old healed-over missing arm THEN. Melissa I assume would quickly figure out Scott was up to something in the process of Derek recovering in his house. Not sure how much would change with Allison/Scott, but he feels awful about not coming back with the bullet and Stiles (/Derek) having to pay the price so he is at least a more attentive friend in this verse. I'm never gonna write more but, that's kinda what I picture. That, and eventual Sterek in this verse.