Until those five words, Sirius had been sure he'd stay. He'd been sure he would protest, refuse to leave things likes this. And then... and then 'if you ever loved me...' came into it. Then Sirius was defenceless. Because gods, he had. And did, still. And always would, despite everything. And right now, more than ever before, he had to do everything he could to prove it.
Last year, probably last week and even ten minutes ago, he probably wouldn't have left. He would have thrown his hands up and said that leaving was no way to show friendship, or loyalty, and definitely not love. But now, Remus's blood was on his knuckles. So everything was different from last year, last week, and ten minutes ago.
Sirius bit his lip and nodded, and said nothing until he reached the door. Then, hand on the knob, he finally found the words, though only in a whisper. "I'm sorry, Remus." He wasn't sure which fucked up mistake he was apologizing for, but he also knew it didn't really matter. "I'm sorry."