When Stiles kisses him, he panics.
They're on Stiles's couch, watching an old horror movie and falling asleep, like so many lazy Friday and Saturday nights in the past six months. Derek expected the evening to end the way it always has before: Scott driving Allison home (check), Danny picking Jackson up to go hit the Jungle so Jackson can pretend he's only going as Danny's wingman (check), Lydia finishing painting her nails and making an excuse to leave (check), and then Stiles eventually dozing off. Usually Stiles stumbles up to bed when he's that tired, but sometimes he falls asleep on the armchair or the couch, and once even on Derek himself.
But instead of drifting off, Stiles kisses him. And then waits, body still right against Derek's face hovering inches away as he waits for Derek to do something.
Derek's heart pounds wildly and he doesn't move, can't think, can barely breathe. When he doesn't react, Stiles pulls away, suddenly putting the whole couch between them, and says, "Sorry. I thought that might be okay."
"It's not," Derek manages to grit out, even if that isn't quite the truth. Because the truth is -- the truth is that he's wanted Stiles to do that for months, but Stiles is so young, and Derek knows how a twisted adult can ruin a kid who's still pretty innocent, and he knows Stiles deserves better. Stiles deserves someone who won't need to be chained up for his own safety, someone who can make him feel good and happy and sexy, someone who can drive him wild without being a wild animal.
But that's what Derek is, at his core: an animal.
So he can't. He knows better, he's been through this before. He remembers Kate's hands on his body and feels sick to his stomach, hears her voice calling him pretty, calling him dangerous and gorgeous and all hers. His memory rings with her laughter, how darkly gleeful she was when he gave himself to her, let her tie him down and use him because he loved her and it was the only way she could be with him.
"I'm gonna... go... go to bed," Stiles stammers, standing, and flees upstairs. Derek can hear him, his footsteps, his heartbeat, his breathing. He can practically smell the shame on Stiles and wants to follow, to beg Stiles to forgive him because it's not Stiles's fault and he'd give Stiles anything he could. But not this. Stiles deserves someone who can love him like Derek will never be able to.
So Derek just sits there for a long time, the TV and the lights turned off, listening until he finally hears Stiles's breaths go even and deep and slow. Then he slips out of the house and walks to his car, gets in and drives away, and he's not sure he's ever coming back.