I'll put it this way. I always knew Darth Vader was a villain in the Star Wars trilogy, but I still fully bought into his redemption arc in Return of the Jedi....and yet we have all the other movies indicating otherwise. Questions of religious redemption and the requirements therefor notwithstanding, how many times does a shady character have to save literally millions of people (or more) before we start thinking he or she is on a path of atonement?
And then I retroactively unforgave him once I saw what he did during the invasion of the Jedi temple in Revenge of the Sith.
People can accept the idea of a character with a shady past who's on a path of redemption, right up until they're confronted with the full reality of that past, in detail.
Or to put it another way, a lot of us will judge someone on the basis of their worst day.