Regulus pulled away when James touched him. It was just two steps backwards, but it seemed a monumental two steps to Regulus. He looked at James almost warily, his gaze full of distrust, because he had just said that they couldn't do this anymore and that meant that James couldn't touch him whenever he damn well felt like it; friends - and especially enemies - didn't just touch each other casually on the jaw.
"I just didn't want to do it anymore," Regulus said, entirely aware that that wasn't a satisfactory answers. But there had been so many reasons that had wove themselves together. "I didn't want to kill anyone," Regulus admitted after a scarce moment. "And he was ruining my family's name."
It was a war that they couldn't possibly win. Regulus had realized that. There were too many lives that were being ruined as a result of it and, in the end, Regulus simply couldn't see the point. He believed in the purity of blood; he simply couldn't see killing families - children - off for it. Even if did actually win, the purebloods had become one of the most hated groups in wizarding society. It wouldn't be a true victory. It would be hollow. Regulus had simply realized that he couldn't stand behind such a cause any longer.