I can't even imagine what Cain's head must be like. "Hey, that person I treated like garbage is attractive now, I need me some of that." And he just does a 180 with no acknowledgement of his prior behaviour. Like, has he no shame? Is personality irrelevant? Since hers hasn't really changed from when he hated her.
The only time I think it'd be okay with someone getting with another person after treating them terribly is something like Koe no Katachi; honest, good-will attempt to make amends with no ulterior motives and the other person is the one to try to start a romantic relationship afterwards.