Sirius was becoming annoyed that the git wasn't taking him siriusly. Or was he simply prolonging his agony? He bit back the retort that rose to his lips, his hands forming fists beneath the folds of his robes, out of sight. He just wanted to get this over with and go on with his life - with Remus. All he wanted was Remus' forgiveness, and this was the only way he could obtain that.
Simply put, he could not live without his friend as a regular part of his life, and he wouldn't, so he'd do what he had to do.
For a moment, he thought about what might have happened - Snape could have died and he wouldn't be going through this mess. He managed to push that thought aside and focus on the present.
"I want to say I'm sorry," he said in a soft voice - after all they were in the library, right? "I never meant for you to get hurt, just scared, that's all. I... I wasn't thinking."
There. He'd said it. He'd apologized. How long would they have to stand there, he wondered. He wanted to leave as soon as possible, but that was up to Remus.