Pangs of conscience, yes. But they do not become good people. They go out to take on the bagmen because they have to leave one way or the other. No, they could not hold Robin hostage. The bagmen would have killed them anyway. Remember that the bagmen do not have the option of allowing them to have the money, because it comes from "the worst" and the whole point is that Chiddick is dead if he gives them the money.
So they go out to the bar. From there, they must decide whether to go after the money or try to slip away in another direction. In the end, they decide to try to take the cheese out of the trap. But staying with Robin and using her as a hostage is not an option. Sarno is the only one of those bagmen who knows who Robin is or, likely, would care enough to put her ahead of his job. In fact, it's hard to prove from the film that even Sarno wouldn't let Robin die if he had to.
But you're right, Painter was in Baltimore, not Atlanta.
Everyone is going to see these things differently, but I don't think you can say that they left Robin at the end out of kindness. They did ask Sarno to let Robin and Painter stay together, but that didn't cost them anything.
"Until that day," in my mind, had nothing to do with revenge. Joe Sarno does not do threats of revenge unless business requires it. He made threats at the motel because his position allowed only that much, but he's a stoic survivor/killer. "Until that day" means that if there ever comes another day when they meet, maybe they can be friends. Those two spend all their scenes together wishing they didn't have to shoot each other.
