Sterek Au: Derek is the one possessed by the nogitsune; it convinces him that Stiles is the one who is actual possessed and the only way to eliminate the threat is to kill Stiles.
“Derek! Derek, please, stop! This isn’t you!” Stiles yells frantically as Derek turns to face him. He struggles against the binds around his wrist, desperate to break free.
“Stop trying to play tricks on me!” Derek growls. "I know you’re not Stiles. This is the only way I can save everyone!“ He turns quickly, scrambling to pull the lighter from his pocket. "I have to end this.”
Stiles’ eyes grow wide in terror. "Derek, listen to me. I am Stiles. You have to believe me!“
The lighter flicked to life. Stiles searched Derek’s eyes, looking for an ounce of familiarity, but found nothing but emptiness.
Derek leaned forward, bringing the lighter with him; the flame licked painfully at Stiles’ neck, ironically making him think of the way it used to be Derek that did the same action. Then, he whispered in Stiles’ ear, "I love you.”