I agree, though I also think there's a discussion to be had about society's obsession with punishment over anything else, and how sometimes it's okay to let go of the past and appreciate that someone has become a better person and is working to attone for that they did and do good things from that point onwards, which is overall better for the world as a whole than them being forced to suffer endlessly for their past actions for the sake of vindication or revenge. If you're going to take the stance that someone can have a moral debt they must be forced to pay, then you have to likewise acknowledge that there must be a point at which it can be paid. If you try to claim that some things can never be made up for and thus some moral debts can never be repaid, then that only highlights the problem with that sort of reasoning. Because if someone takes a life then saves a life, and you claim that one is not enough to make up for the other, then you're essentially treating life 1 as more valuable than life 2. And what if they take 100 lives but save 1000? Can human lives even be stacked up against each other like that to say which group has more "value" than the other? That's the paradox of a moral debt, something can not simultaneously be priceless and yet also not hold enough value to balance the scale against itself at the same time.

In real life this can be complicated further because it can be hard to judge whether someone has truly learned from their mistakes and genuinely changed their ways, but in a fictional story you often get to see for sure that the character truly is sincere. So to tie that in to what you said, just because a viewer/reader is capable of accepting a character's redemption in a fictional setting, where they are 100% certain that the former villain has had a change of heart and feels bad and will continue to do good things into the future, that doesn't mean it's a moral failing on the audience's part. But it's also worth noting that being willing to give someone a chance to improve themselves and grow as a person instead of demanding their eternal damnation and punishment isn't a moral failing either even outside of fiction.