Derek had to stop, remind himself that this thing isn’t Stiles anymore. The fiery boy that was inside is long gone. This thing, with the lifeless brown eyes that used to shine with defiance and intelligence, is ambling towards him and he keeps telling himself ‘this isn’t Stiles’ over and over. Knowing that doesn’t make it any easier when he plunges the knife into the body that used to be his soul mate, doesn’t make the tears easier to choke down. “I’m so sorry I couln't save you." And he was.