Yay what a fun topic!
I think the appeal of a villain romance can be the forbidden love angle. The “I shouldn’t care about them, but I can’t help but be drawn to this person.” The angst that can accompany that can be compelling. Additionally, there can be an air of tragedy around it: “if only things were different,” “if only we had met under different circumstances,” “maybe in another life we could have been lovers.” So much angst to enjoy!
As to the redemption arc question, I think the frustratingly ambiguous answer is it depends. I think it is heavily influenced by what bad things the villain has done. Has the villain literally killed the MC’s family? Okay, big yikes, seems like we are going to need to do a lot of legwork to make it believable that the MC falls in love with them. That might involve a redemption arc, or the two are forced together in some way, or they just have such amazing chemistry together, etc. etc. I think though that this is where the angst/“I shouldn’t like them” angle that people like comes in. A redemption arc would take that away, which is probably why people aren’t always the biggest fans of them. That and the fact that redemption arcs can feel really out of place or half-assed. Tone and genre matter a lot here too.
In regards to the toxicity question, other commenters have already pointed out that just because someone is a “villain” doesn’t mean they’d be a bad romantic partner. As a related example, I once watched a show with an Evil Crown Prince who was power-hungry, greedy, and cruel. But he had a sister he genuinely loved and was very sweet with. He wasn’t faking it, he wasn’t trying to manipulate her; he really cared about her. But that love didn’t outweigh all the terrible things he did. Here’s the tragedy again: if only he was as kind and loving to others as he was with his sister. I hope that’s a helpful example to demonstrate how a villain character can be capable of having loving relationships while also committing horrible acts.
Wow that was long, but it was fun to write. Hope this has given you some food for thought!