He hadn't had a choice. She was going to kill Scott if he didn't kill her. And she just had to use a sword didn't she? So when he ran her own weapon through her, it was like reliving killing her.
She was just another of the assassins that had come to Beacon Hills, and now Stiles' knees were going week as he looked into her dead eyes and saw Allison's instead of the blue ones that were actually staring lifelessly into his.
Might make a killer out of you yet. Although, that already happened didn't it? A voice in his head asked him with the same coldness the nogetsune had spoken with.
Scott caught him by the arms as he started to fall, bringing the girl attached to the sword in his hands, still holding her up, down with him.
"Stiles? Stiles, are you okay? She didn't hurt you, right?" Scott asked worriedly, his voice sounding distant in Stiles' ears as his breathing became more difficult.
"Oh fuck. I killed her. I killed her. Oh God. I'm sorry. I killed her. I'm so sorry." He was saying quietly, not even aware of the words leaving his mouth, because all he could see was Allison staring at him with dead eyes.
"She was going to kill me. You were just trying to protect me." Scott said, prying the handle of the sword from Stiles' grip, causing the girl to fall to the wet ground with a thump of her body and a clatter from the sword.
Scott turned Stiles to face him, get him to look him in the eyes.
"I killed her, Scott. I'm so sorry. I killed her." Stiles whispered, looking at Scott, eyes filled with unshed tears. Even Stiles wasn't sure which he was talking about at this point. Allison or this girl, whose blood was mingling with the puddles on the ground.
"You were just trying to protect me." Scott said firmly, pulling Stiles to him, letting Stiles break down with his face pressed to Scott's shoulder.
Tears won't bring her back. I'm sorry's won't close up that hole you left in everyone by taking her away.
The shaking in Stiles' body got worse. Scott was holding him to himself, running his hand through Stiles' hair. Scott was his grounding point.
Why would Scott want to be near you? Why would he want to be friends with the boy who killed the love of his life. It's a wonder he can look at you.
"It's going to be okay. Everything will be fine." Scott whispered to Stiles, trying to calm him down. "There was nothing you could do."
"Y-you're right. Sorry for freaking out on you. Shock, I suppose." Stiles replied, mustering up the will power to stop crying and lift his head off of Scott's shoulder. He shouldn't be doing this to Scott. Scott was the one to loose Allison. He had no right to be the one crying on Scott's shoulder.
"You don't need to apologize." Scott told him, pulling Stiles close again when he started to pull away.
He knew telling him it was going to be alright and hugging him wouldn't help Stiles, but he hoped making the effort would show Stiles that he didn't blame him. And maybe, eventually, Stiles would be able to stop blaming himself.