Because. Because he was made into who he was. Because he was, like, my age or something, and we're supposed to be saving these people, not killing them.
[... a pause.]
That's what Scott will say, anyway. [And that's the thing, here: Stiles might not believe in Scott's philosophy entirely, might be more willing to let people die when they deserve it... but if this will put him against Scott? No, never. He'd never do anything like that, except he had to, and now he's screwed.]