Regulus had tried to pull one over on Voldemort? That bit was news to him, and Remus's eyebrows shot up, as though attempting to blend into his hairline. As far as reasons to get killed, that was a very good one, and completely unexpected. Part of him had to wonder if James's declaration that he was going to get himself killed might have played a part in Regulus's decision to turn against Voldemort (and thereby played a part in Regulus's death), but he had the common sense not to mention anything. The question and the possibility remained, however.
"You may have been right, but... He's the only one who can forgive you, isn't he?" Remus asked, hoping that James would get the point rather than argue against it. James could be very, very stubborn when he wanted. "You're obviously not going to be forgiving yourself, and there's no one else. Not for the punching, I'm sure he deserved it. But for not helping. For not doing enough. Regulus is the only one who can forgive you for that. If he does, you can put it behind you. If he doesn't," which Remus suspected was rather more likely given that he was a Black and he had been punched and he had been killed, whether as a result of James's inaction or not, "you'll know to work to make it better between you two. To work at a... a sort of relationship. Maybe a friendship, if you can get that far. I think you should talk to him, at the very least. Let him know that you regret it."