I feel like most writers and readers misunderstand what a redemption arc's purpose should be. Too many stories focus on the villain having somehow gained enough redemption brownie points for the good guys to forgive them, which to me also shows a lack of understanding of what forgiveness is about.
Most villains in stories don't really go through arcs. They're already set in their ways and usually aren't open to change. That's what makes villain redemption arcs interesting. It's one of the avenues that you can take your villain to experience character development and growth as if they were one of the protagonists. Having them be forgiven by the protagonists should be a side effect to their arc, it shouldn't be the main purpose. And hell, you don't need the protagonist to forgive them for it to be an effective redemption arc. Example: Look at Scar from FMA.
In my opinion, it doesn't matter how bad the villain actually is or whatever atrocities they committed, if your main focus is to write a good arc, with no main goal to have them be forgiven at the end, then I'll say you are free to write whatever redemption arc you desire. The villain should pursue this arc for their own development, not to seek validation from the heroes. And the heroes on the other hand, should only forgive if they themselves think they are ready to forgive for the hero's own sake. No one deserves forgiveness, that's only for the victims to decide if forgiveness is what will save themselves.
Basically, your main goal for a redemption arc should be to write an interesting arc for your audiences' entertainment. Yeah, it might even provoke a negative reaction or two, depending on how far in the villain scale you have them at, but again, you shouldn't be writing this to make your audience forgive them or make the audience love them. Remember, you're writing this purely for the character's sake and their growth, no one else's. No one has to like the villain at the end of the redemption arc, the only thing that matters is what the villain goes through and how they grew from it.
So many redemption arcs fail because they don't understand this fundamental goal of what a redemption arc should be. So many stories just use redemption arcs because the writer wants to backpedal on their villain being...well...bad, and want them to join the good guys instead. And the good guys somehow have to forgive them after said villain did like, three good things and apologized. Also oftentimes, the villain never even struggled with their ideal being changed to good. They just flip a switch in their mind after maybe a talk no jutsu that they suddenly see the error of their ways and now they are good cool people. Throw in some retcons here and there about how villain didn't ACTUALLY do some bad things and bam now you can make villain join the good guys without making it too awkward right? Right?