The Doctor might have stopped himself, but Jack heard it. He heard what the Doctor was saying. Vengeance. Yes. And Jack had wanted that too, hadn't he?

"You know..." it was hard to say anything at all, because it was hard to even admit. But then maybe he had to.

"It wasn't just-- the reason I never left the TARDIS. I think... I think I'd have gone too far. I think I still could."

He turned then, looked at him. "I think we both could. We need to make sure we don't."