Regulus flinches but it's not because it's Sirius - he's too used to the torture now, still too pained from the after-effects. But he relaxes into it, shaking his head and burrowing it into Sirius' shoulder. "I fucked up," he whispers, barely audible.

"If I'd been able to derail her enough she wouldn't have escalated, it wouldn't have gotten so bad--" At Sirius' question his head snaps up to stare at him, eyes wide. "She really would have killed both of us then," he says, "that curse doesn't work if you don't really hate someone. It... It would have failed, and she would have known I was lying about just doing it to be a spy."

Ah. So that's it.