To me, all redemption requires is inner change--becoming the sort of person who won't do the things you did before. Atonement and sacrifice are things that may flow naturally from that change, but that depends on exactly what you changed to and from.
Incidentally, I'm a big fan of Superior, a manga about the gradual redemption of a genocidal mass murderer. I think it's a little too soft in parts--change shouldn't be quite this easy--but overall, it does pretty well at documenting a journey from villain to hero.
Yes, I could see that working. I think, as you said, the main problem would be being subject to a charge that it's too easy. For a genocidal murderer you'd have to have a very convincing and, in my view, long transition process.