"He's - no. No he can't - " Derek was dimly aware that he might be in shock right now but he couldn't deal with that at the moment. Everything had fallen out from under him. "Stiles don't point that thing at him, he's - he's not well, he's just - you're sick, right Peter? They manipulated you to do this, you would never side with them, you aren't - "
Peter chuckled and shook his head. "Oh my innocent little nephew. The little bunny of the family. Our little Der-bear. You don't understand anything that's going on. The muggles and mudbloods are an unfortunate boil on our magical society, and it's time that boil be drained before infection sets in. We need to purify ourselves once again. This is only the start, Derek, and it will go on and on until every single last one of them is wiped out. It's for the best, Derek." He reached out and snatched Derek's slack hand. "And we'd love for you to join us. We could benefit greatly from one of your kind being on our side."