Prompt: Castle kills Tyson (premeditated) and confesses to Beckett. Instead of being appalled and turning him in, she's turned on by it.

Not many people know that it takes an average of three minutes from application of force to death from strangling. Unconsciousness happens quickly – if the victim is lucky – but death is slow.

Richard Castle knows.

It's happy accident. A once-in-a-lifetime opportunity. Eight million people in this city, and who does he see out and about with all the other monsters on Halloween, not even bothering to wear a mask? Not that he needs it – Castle's been writing and hanging around murder long enough to know that the worst monsters are only human.

Right out there in broad daylight, a sick fantasy that's lived in the dark recesses of his mind for years at last has the chance to bear fruition, to become.

He was on his way home after popping out for some last minute party supplies. The guests will be arriving in two hours, costumes mandatory. He made sure Beckett understood the mandatory part, and oh he can't wait to peel whatever costume she's come up with off her (damn woman won't tell him, but Ryan and Esposito know and they said he'll like it, which just makes waiting that much more unbearable) and celebrate their first Halloween together. The night they'll announce their engagement to their friends and the public at large.

His mind is made up.

Castle hopes he'll be celebrating something else entirely tonight too, if only in his own mind. The freedom from the fear, the chance to live no longer haunted by the every shadow on the wall or footfall in the night.

Martha has done a bang-up job on his makeup. His ruggedly handsome features have been temporarily morphed into something well and truly grotesque, reprising his starring role as zombie.

The soul-dead serial killer walks right up next to him, stands on line at the news station and buys a paper, even makes almost-normal conversation with the guy manning the stand as Castle busies himself with his phone six feet away. Tyson saunters off none-the-wiser, and Castle almost can't believe his good luck.

(Predictable, Jerry; so predictable. I'm almost embarrassed for you. People don't buy papers far from their own homes, now do they? That was your mistake, Jerry. That will be your undoing. Buying a paper, trying to pass yourself off as a human being.)

Adrenalin rushes through the author, his hands clenching and unclenching reflexively. Heart racing even at the casual pace he keeps behind his mark, the anticipation of the kill unexpectedly thrilling. His shoulders roll ferally as he stalks - the picture, to any onlooker, of barely-controlled power, holding himself back on a tight leash from attacking before the moment is right.

He's written this scene a thousand times in his head. The USB he keeps hidden in his desk once housed the Beckett file, but with Senator Bracken's fortuitous run-in with a car bomb, he's replaced it with a dozen Word files. Stories he'll never publish about a certain reporter exacting some kind of gruesome revenge on a serial killer whose personal mission is to destroy the life Rook's trying to build with Nikki and their family.

Until now, he's always told himself it was just catharsis, a creative outlet for the frustrations of having no leads on Tyson. Indeed, no contact at all since the night at the bridge. Beckett says he's paranoid, when he occasionally has Ryan sweep the loft for bugs or cameras, or rants to her about how it could have survived and is probably watching them, waiting for them. Deep down, Castle thinks she knows too. She just doesn't want to admit it. Besides that, she doesn't have this primal need to see Tyson's lifeless, miserable shell and know it's finally over. Not like he does. It's his quest, his dragon, not hers.

It became personal when Tyson hurt Ryan and threatened him, taunted him, dared to compare Rick to himself. It became a mission when it used Ryan's gun to try to set up the younger detective, the brotherly affection in him roaring at the manipulative invasion. It became an obsession when it dared touch Beckett, dared to try to use her against him. No one threatened Beckett. No one watched them, invaded their sanctuary, and got away with it. No one dared to taint the memory of something as beautiful and perfect as their moments alone.

Looking back on it now, that was when he knew. The moment he found out Tyson had been in his home, watched him with his family. He knew. It's a cliché of the Chosen One heroic tale genre: neither can live with the other survives. Something like that.

There was a time when he thought Kate Beckett was possibly unstable, for how much her mother's case drove and consumed her, how frighteningly obsessed she was by it. After Tyson, he understands. He knows there'll be no peace for him or Beckett or any of their family (blood or otherwise) or safety for anyone else while Tyson lives.

He knew he'd ultimately end Tyson. He just always figured it would be a case of self-defense, not an execution.

Jail is not enough. He realized that some time ago. Merely catching Tyson would do little good. The thing would act through someone else, or escape, no matter what failsafes the prison system thinks they can place on it.

Now, he knows for sure. All that writing and thinking and – dare he say it – fantasizing, was no mere catharsis. It was a rehearsal. A solemn oath, a manifesto.

He has the file where Rook kills famed serial killer Terry Bryson with a shot to the back of the head after finding Bryson lurking in the apartment, feet away from a sleeping Nikki. He has the file where Bryson confronts Rook in the subway, a struggle initiated by the serial killer ending with Rook pushing his nemesis in front of a speeding train and watching with satisfaction as the wheels saw the miserable body in half as dozens of people look on, and isn't that just a little exhibitionist?

And then he has his favorite file. It's the one he secretly goes back and reads when he's restless and worried about Tyson as Kate sleeps peacefully in his bed just feet away. The one where Rook turns the tables on Bryson and stalks the stalker, follows the beast back to its lair, and puts it down like the piteous animal it is. He likes that one best. Castle finds something so very poetic about letting Rook kill Bryson in the beast's own inner sanctum, after having his and Nikki's privacy so abhorrently invaded.

Life, imitating art? Perhaps. It's almost eerie how well this fits with his stories, but truth, as they say, is stranger.

Three blocks over from the news stand, Tyson gives a furtive glance around, as if to check for anything unusual. But it's Halloween, and everything is unusual. Dark, rodent-like eyes dart about nervously, scanning the street, as if the being has an unnatural sense to know when it's being followed. Castle hangs back in the middle of a pack of werewolves like he belongs there, like it's all part of the festivities.

Following his target over the threshold, he watches the thing pass through the lobby (no doorman, excellent) and into the far-left elevator, the "4" lighting up above it as the doors slide closed. Castle's mind whizzes: left side of the building, fourth floor. Can't be more than six apartments in that wing. It's figuring out which one is Tyson's that's the challenge.

He rides the elevator up with Buffy and Spike, too preoccupied making out to notice him, and the guy from that zombie cable drama who jokingly points a toy crossbow at him. Castle smirks in return. An elevator full of monster hunters ascends, cheerful bubblegum pop playing over the polite silence of the cramped space. Buffy and Spike depart on 3, and the zombie hunter is waiting for 6. Castle departs with his best zombie growl that seems to delight the crossbow-wielding partygoer.

Seeing no one in the plain but well-lit hallway, Castle decides to use the holiday to his advantage and just go with the brute force approach. Try doors until he gets the right one.

Steadying himself – easy, Rick, you only get one chance at this – he knocks on the first door.

Can I help you? a young woman says through the cracked door, chain still latched.

Oh, he feigns surprise, this isn't the costume party? My mistake.

The woman sighs with relief, opening the door fully as she chats happily and says that the Pierce's party is at 6B, not 4B. She smiles and flirts, simpers, I like your costume; maybe I'll see you there later, as she closes the door.

He's just thinking about how he can use this information when another door creaks open, two down.

Tyson doesn't connect it immediately and it buys Castle precious time, his feet carrying him swiftly. The smaller being seems to realize what's going on as Castle meets its beady eyes, already looming over it as Tyson tries to slam the door, reach for a weapon, anything. Tyson is no sucker, but Castle is much, much bigger and stronger.

The rage, hatred, fear, and pure desire for revenge give him an even greater strength advantage, the extra energy fuelling him and making his brain and vision swim deliriously, blood rushing in his ears, heart in his throat, a deranged high coursing through his veins. On their own accord, Castle's hands wrap around his nemesis' neck, steering it back into the apartment without a sound.

Tyson fights like a trapped animal, thrashing, landing punch after punch to Castle's chest, but it's of little use. Castle's had worse, and besides that, he's even stronger than he looks. Though he prefers not to throw his weight around with suspects, allows the real cops to do their jobs without his potentially legally-complicated involvement in arrests whenever possible, he holds his own when there's need of it. The adrenalin coursing through him means it won't hurt until much later. A knee pressing into Tyson's chest, immobilizing it easily, Castle adjusts his large hands around the neck of the hideous creature and chokes.

There's something incalculably satisfying about it, as the seconds tick by, there on the ground in the entryway to Tyson's abode. He plays with his quarry like a young coyote with a hare, thrill zinging through his body, making his blood sing as he edges his prey close to unconsciousness, then eases up just enough to revive awareness. He idly wonders if this is what being god feels like, as he makes absolutely certain this forsaken thing knows fear in death as it never knew in life.

Tyson's punches grow weaker, then stop falling altogether. The face turns a fascinating white, then red, then bluish-grey as inky eyes bulge and loll crazily in their sockets, the only noise in the room the occasional wheezing gasp, the rattle of death.

Castle's only watched a person die up close once before. Both times, he hovered overhead, watching the whole thing. It's a strange symmetry, watching Beckett die versus watching Tyson die. The person he loves most versus the only thing Castle can honestly say he's ever hated. The first time, every second was a desperate plea for time, for another chance, for a miracle, whatever the price. This time, he's making certain there will be no resurrection, no miracle for the damned.

"How close… to death… do you want to get, Jerry?" he taunts, his breath heavy with exertion and unhinged ecstasy.

Watching the light die in the madman's black, abyssal eyes as his thumbs crush Tyson's windpipe and cut off the last vital bit of oxygen to his brain, Castle feels a rush of satisfaction, excitement, relief, vindication. He's free. They're free. He and Beckett can move on with their lives, can be a family, can have a family, and live free from having to look over their shoulders for Bracken or Tyson. He holds for a minute longer, making absolutely certain that the walking nightmare is gone.

Rising at last, he takes a deep, steadying breath and begins robotically cleaning up what little of a scene there is, his many years with the cops and knowledge of CSU and M.E. procedure finally coming to real-world use.

It's over. It's gone.

What took you so long? Frankensito greets as he walks in the door of the loft, seeing it transformed by his patchwork family into Halloween central.

Place was an absolute zoo, as you can imagine, Castle is ready with the lie and spits it out easily, having anticipated the questioning. Had to wait on line forever, then I almost got home and realized I forgot to buy dry ice, so I had to go back.

Frankensito shrugs, satisfied with his answer, and Ryan – or the Artist Formerly Known as Ryan, if his costume is anything to go by - bounds up to talk to him about cocktails from behind the kitchen counter where he and Jenny/Madonna are preparing hors d'oeuvres.

An hour later the party is in full swing, the loft filled with people. Kate's hanging close to him in her lovely Indiana Jones costume, sinfully ill-tailored for fighting Nazis or running from booby-trapped temples. She even has a bullwhip.

Castle's not able to enjoy any of it. Panic claws at his chest, guilt flooding him, dragging him down and filling his mouth and nose and lungs.

Not about killing Tyson – that thing needed to be done away with – but about lying to Beckett. About what it'll do to her and Alexis and his mother and Ryan and Esposito if this comes back to him, if he has to go away. About the pain of a trial, about some D.A. or his attorney making Beckett testify for or against him. About the wedding they'll never have, the family they'll never have.

Small-talk with Gargoyle Markway fails to hold his attention.

Heard you brought back Derrick Storm?

Yes, for now.

Good, good.

How's your wife?

Good, good. Care to stake some of those royalties on a game of golf with Bob next weekend? We like you. You make us look like we know what we're doing.

I'll see if I can clear a spot in my schedule.

Castle escapes as quickly he possibly can without attracting attention, makes for his bathroom, bile rising in his throat as he drops to his knees with a clash he feels in his bruised ribs. His insides become black and slimy writhing things of the murky depths as he bows his head, feels the burn up his esophagus, and vomits.

Dizzy, he presses his face against the cool of the wall, sagging back against it and leaving black and green streaks of makeup against the pale subway tile where his cheek rested.

"Castle?" Beckett calls.

Shit.

"'m fine," he groans weakly, but upon seeing her stand in the doorway, his stomach flips all over again.

Humiliation mixes with fear and guilt, her seeing him like this. How could he be so rash, so stupid? A few minutes of satisfaction, even if they lead to the safety from Tyson that's now theirs, they're not worth what he's just thrown away. Her love, her trust. The security of Alexis and his mother. The only real friendships he has – real smart move, Rick. Kill someone when your best friends are cops and are likely to catch the case once the body's found.

They won't roll on him, he knows that much. But they should. Dear god, they should; they shouldn't have to cover for him, lie for him. They already carry Montgomery's sins in their throats. Somehow, the fallen Captain's secrets haven't been exposed even as the late Senator's secrets continue to come out, a posthumous parade of corruption and deceit. They shouldn't have another mouthful of lies to swallow, and he vows to spare at least Ryan and Esposito, if he can. They don't need to suffer for him.

"Castle!" she snaps fearfully, he knows he must be drifting in and out. Her voice sounds hollow and far away.

"Kate…" he mumbles, still swimming, still underwater. "Oh, Kate, I need to tell you something."

Kneeling down next to him and rubbing the back of his neck with a tenderness that makes him sick, she waits patiently. "Tell me, babe."

"Not here. Can you…" he thinks fast, trying to come up with a plan, "can you make my excuses to Mother and ask Jenny and Ryan to take over hosting? We need to go to your place."

To her credit, Kate doesn't bring up their definitely-not-happening-tonight announcement, sensing the gravity of this information.

"Okay," she takes a deep breath, "okay. Can you stand up?"

Castle hauls himself by the counter and sways a bit, but he's sturdy enough.

"Yeah, I'll be okay. I'm going to get some water and clean up."

Gargling and brushing his teeth thoroughly, Castle scrubs the makeup from his face, pulls and combs his hair back into something like normal, and after drinking some water, he feels a little calmer.

They manage to escape the party with little more than a concerned feel-better-bro from Espo and a reassurance from Martha.

I'll have the guests out at a respectable hour: two, three in the morning, tops, she says airily, her Harley chick bleach-blonde wig flipping as she turns back to the party and they slam the door behind them in their desperation to get out. See you in the morning, darlings!

The subway ride is silent.

They kick off their shoes at her door and she takes his cold and clammy hand in hers, sliding his shambled coat off his shoulders and dropping her hat on top of it in the foyer.

"What's wrong, Castle?"

Best to just get it over with. Spit it out. The sooner she hears, the sooner she arrests him, the better.

He says it flatly, emotion sapped from him as he gives everything he has up, lost on a momentary whim, the final victim of his own dark impulses and ill-conceived protective desires.

"I killed Tyson."

Note: Will be a two (maybe three) parter. Since this is Kink Meme, porn is a promise.

This is wayyyyy out of my comfort zone, thematically and stylistically, so I'd really love to hear what you think. As always, comments, questions, concerns, complaints and constructive criticisms are very much appreciated and responded to.