Remus would have become a killer last night. If James hadn't intervened, hadn't saved Snape's sorry life, how much blood would have been shed? Sirius doesn't care about the greasy bat's well-being, but Remus... If he'd lost control and done the unthinkable, he would have been broken completely.
And it would have all been Sirius' fault.
OoOoO
He can see the anger in Remus' eyes when he goes to visit him. It lingers beneath the tiredness and shock, fierce and blazing. But Remus doesn't yell, because Remus would never raise his voice, would never lash out, even if he has every right.
Instead, he holds Sirius close, as though he's trying to protect him. It should be the other way around. Sirius should have protected Remus.
And with that thought, with his failure, Sirius breaks in his lover's arms.