[He listens this time, really listens properly-- it's hard, because he doesn't want to, because he wants Sirius to stop acting like this, he wants everything to go back to normal, he wants to not believe-- but there's something bleak in Sirius' expression now, something he can't quite ignore.]

We can't.

[He says that bleakly, quietly.]

We can't kill him, Sirius. He's our friend. And he's . . . he's blaming himself, he's wracked with guilt-- we'll figure something else out. Anything. But we can't kill him.