Redemption arcs in villain romance get me every time, but they're a tricky balance. The shift from monstrous to morally grey has to feel earned, not just a magic wand waved because the protagonist is 'special.' A book that nailed this for me was 'The Unseelie Prince' by Kathryn Ann Kingsley. The guy is genuinely terrifying and cruel, and his affection for the heroine doesn't erase that; it just complicates his worldview. The change is incremental, messy, and never fully absolves his past.
Some readers love a full heel-turn, but I'm more invested in the ones where the antihero stays dangerous. Like in 'Land of the Beautiful Dead' by R. Lee Smith. Azrael is a literal death god, and his 'redemption' isn't about becoming good. It's about being worn down, changed, and finding a reason to be something else, even if that 'something else' is still terrifying to everyone else. That feels more authentic to me than a complete personality overhaul.
Honestly, I get bored when the villain loses all his edge by the third act. The best ones keep you a little scared of him, even when you're rooting for them together.