The way I see it, there's two things going on. There's the creative side, where any writer can give any character they want a redemption arc. The only question there, for me, is: in terms of writing skill, is this arc well executed? Does it build well? Does it mesh naturally with the overall themes of the work? Does the psychological portrayal ring true as something possible within human experience, even if it's not likely or ordinary? It's understandable to argue over these things, because our experience of fiction is subjective, but it's not understandable or acceptable to put a "moral question" here. I believe in freedom of expression: a writer can redeem any character they darn well want.
The second thing is the reception side. This is where morality can come into it IMO. Where people can ask: do I, personally, find this character too distasteful to even want to consider seeing them be redeemed? Or: do I personally have red lines a character cannot cross for me to want to see a redemption arc? Or even maybe: do I find there something socially harmful in this depiction of a redemption arc? Everyone will have their own answers and that's okay. What they can do is vote with their $$. Give their reasons for their personal objections. Maybe even find likeminded people and all vote with their $$ together or write in to the studio or publisher or something like that. But people who enjoy it can also vote with their money! And write in to say how much they appreciate it.
What I see among antis is a conflation of these two different levels, though. So they take their subjective feelings and say that it's somehow objectively bad writing or harmful politics and therefore should be disallowed for an artist to do something if it crosses those personal lines. Or that other people are bad people if they appreciate a well executed redemption arc and see the moral and political implications differently.
That isn't how art or ethics works. That's how censorship works and it is, if you'll excuse the vulgarity, bullshit.