By killing him. By killing Peter.

[He stares at Sirius, breathing sharply. His eyes sting a little, and he blinks hard, trying to will away the tears. Now isn't the time for them.]

Don't you think I want to keep him safe as well? Don't you think I'm frantic with worry? I'm still his friend, you know. Even if I'm not-- I'm not his best friend, he's still my mate. But Peter is as well, and-- and condemning him for a crime he hasn't yet committed is low. It's bloody awful, and I won't do it, much less let you or anyone else kill him. There's other ways.