"I CAN'T, god damn it!" Peter shoved Sirius so that he stumbled and landed on the couch; it wasn't intentional, he just needed Sirius to get away. "You can make mistakes! You can do things you know are wrong! You can take risks, because when you wake up tomorrow, no matter what stupid, selfish thing you did, you know you're a good person! I don't have that!"
Peter looked away, unable to meet Sirius' eyes. "I can't... risk it. I can't let myself be selfish enough to become... him. And I'm not going to let you ruin anything with the others just to be with me." He walked away, his flat too small to effectively storm out of any room, and took his coat. "If you won't leave, then I will."