My contribution to the bot!Sam genre.

Pretender

"Bumblebee!" Sam yelled, skidding to a halt so fast he over balanced and fell. The Decepticon laid another blow that sounded like tectonic plates colliding on the already-down Autobot. Sam scrambled to his feet, hands scrabbling, slicing open on the ground, and ran back.

Bumblebee saw him coming, his one working optic widening in horror and his damaged body thrashing weakly as his charge and best, human, fragile friend barreled right back into danger.

The Decepticon ignored Sam like the insect it considered him to be, too used to battle to give his downed enemy a chance for one last shot. It growled when Sam took a running leap and landed on his back, disgusted as the human clung and clambered like the simians all humans shared DNA with.

Then Sam reached his neck and, in a very human show of violence, brought his fist back and plunged it forward to strike.

The instinctive, mindless attack born of fear and rage would have been a hell of a lot less effective if, with a familiar scrape of sliding metal and whining energy, Sam's arm hadn't refolded itself into a shining blade of Cybertronian alloy.

Like a needle sliding through skin, Sam's arm slid through the gaps in the Decepticon's armor that no bot's fingers could fit into. His blade-arm slid through wires and tubing, nerves and arteries, until the end of it – curled back and serrated – caught on something and on instinct alone, Sam twisted.

The Decepticon screamed, utterly unprepared, utterly fucked. He was already dying before Sam's blade lodged in one of his primary relay circuits and destroyed it, cutting off 80% of his body control. By the time Sam wrestled his blade-arm out and dropped away, the 'con was on his knees, gushing energon, oil and other alien fluids whilst ugly sparks jerked his body around from the inside.

By the time he fell, face-down, he was dead.

Breathing hard and feeling a little like he was coming down from something narcotic, Sam turned to Bumblebee. The yellow bot had turned onto his side and was staring at him in absolute shock. At least he wasn't bleeding/leaking anymore and even as Sam watched, the Autobot's second optic flickered back into unsteady life.

With Bee visibly not about to die, Sam's mind allowed him to take a look down at the hallucination he was suffering.

The blade, extending smoothly from his elbow and streaked with Cybertronian fluids, glimmered smugly up at him.

Sam had just enough time to realise he was hyperventilating before he blacked out.

Pretender

"I don't know, Bumblebee. As far as I can tell, there's nothing. He's human. If you hadn't provided a copy of your memory file, I wouldn't have believed you for an astrosecond."

"I get it, Ratchet, but... he was terrified. What am I supposed to tell him?"

"The truth, however unpalatable, would be best."

"What he said." Sam seconded groggily, blinking awake to the harsh lighting of Ratchet's mech-built med-bay. It was, weirdly, kind of comforting. If he'd woken up in a human hospital, he'd have been waiting for Sector Seven to kick in the door and kidnap him. But there was no way Ratchet would put up with that shit in his med-bay.

There was a short silence before the whirr-click of softly-stepping Cybertronians came towards him, thankfully blocking out the harsh light as they bent over his berth.

"Sam, how are you feeling?" Ratchet asked briskly, scanning him as though he hadn't probably already scanned him in every possible way whilst the human was unconscious.

"Um."

Sam glanced down at his arm, which was innocently pink and fleshy and not at all metal-killer-of-Decepticons-y.

"A little trippy?" He asked hopefully. Maybe Leo had slipped something in his beer, the guy was always going on about how Sam needed to loosen up, even though Sam was practically the poster boy for slacker student.

"I found no foreign agents in your blood." Ratchet shut down that option. "Nor can I detect any chemical imbalances in your brain, although I admit I haven't any prior scans to compare them to and must base my estimation on the inferior information humans have collected.

Sam swallowed. "So," he said, would-be casually. "anything that explains my arm..." He stopped speaking, almost physically unable to say it.

If he said it aloud, that would make it true. That would make it undeniable.

That would make Sam... Not Human.

Pretender

So, Sam's origins were pretty cliched. My thinking was that the Allspark was way more powerful than some stupid spark-destroying blaze of light. The energy contained in it had to go somewhere. That somewhere was Sam, changing every molecule of his being during those few seconds, without him noticing.

Sam-as-Pretender continued his life, never realising that he was no longer human. Nobody knew. Sam's model was built by the Allspark and is extremely sophisticated when it comes to 'pretending'. Not even Ratchet's medical-grade scanners can see anything non-human.

Most of Sam's system processes run in an artificial subconscious mode and a lot of them are dedicated to replicating Sam's human manner of thinking and feeling and being.

The system will either maintain Sam's appearance as he perceives himself, or will run an aging program based on a human's life cycle. However, with Sam now being aware of his own nature, his systems may belay the aging illusion.

Sam is unable to voluntarily transform or access his own systems due to his own fear and desire to not be a Cybertronian. He was born human and he wants to stay human. In changing species he suddenly sees everything he knows – all comfort and familiarity and belonging, slipping away to a colder, starker existence as a being trapped in a race perpetually at war. Everything and everyone he knows is doomed to move on without him.

The sort of mental shift necessary to be fully Cybertronian may never be possible. Pretending to be human, even to himself, is as much a survival instinct as the Cybertronians taking alt forms when not in actual battle. If Sam's systems hadn't evaluated the risk and set up the 'secondary OS' in his head, he probably would have gone insane at best or self terminated at worst.

This story never got developed further because I just wanted pretender!Sam who had to continue pretending to be human in order to stay sane, despite knowing differently. I couldn't quite find the end of the story so it's being posted here.