The difference between Sarah and Zero is that Sarah started to change only after she had no other choice. Her change of heart isn't organic and doesn't come from a place of real regret. it comes from having everything taken away from you and having no other choice.Ehh. I don't think the point of redemption is that you can only be forgiven for past misdeeds if if you did them out of complete ignorance and borderline lack of free will.
The core principle is regret. Do you sincerely regret your actions? Are you willing to make amends? Then yes, you should be given a chance to start over and be a better person.
Sarah's "evil" was petty at best, and she was basically a spoiled teenager nursing resentment against her weird possessed sister and a father who spoiled the shit out of her (and who she may have suspected deep-down wasn't her real dad). That's not really something that should place redemption forever out of reach. If you were a bully in high school you shouldn't be treated like a monster for the rest of your life, especially if you mature enough to realize that what you did was wrong and you feel bad about doing it. In the end Sarah chose to be good (as is mentioned, Eris turns against her because she's not willing to murder someone), and she's been slowly overcoming her old bad habits and becoming a better person. Is her road to redemption finished? No. Should she be forever prevented from ever being redeemed? No.
There are plenty of things that should never be forgiven, or at least not forgiven without MASSIVE sacrifice (which is why "Death by Redemption" is a trope). But petty evil mostly spurred on by being a spoiled child and traveling in social circles that only reinforce that mindset isn't really something that should damn you forever.
And honestly, one could potentially argue Sarah was just as brainwashed as Zero in a way - except in Sarah's case it was society itself that made her that way and not a secret criminal organization that trained her to hunt down and kill its enemies without remorse. About the only thing that makes Zero's story less morally dubious is that she's essentially a failure, and never got around to actually successfully killing anyone (though she would have, if Scarlet's power hadn't protected her). If she was better at her job she'd have a lot more bodies in her past to make up for. Like Jeanne, who has killed or captured a lot of innocent people - we just tend not to hold it against her as much because she's always been nice to us.
Which I think is the root of most hate for Sarah. We got to watch her being a shit to us (and to sympathy-bait like Alice and Sonya), so it's easier to resent her for it than it is to resent characters who've done most of their bad behavior mostly off-camera. But Sarah's also gone through more abuse/punishment in the wake of it that any other character, so it kind of balances out.
Sure. Zero has a similar situation. But unlike with Sarah, where the story is focused on what she has done, with Zero the story focuses on whats been done to her.
Sarah was a total bitch because she could. Zero because she was made to be.
Or to put it in another way: Sarah had a choice all her life. And she chose to be a bitch until the choice was taken away from her. Zero never had a choice, until now... and she is coming around. Fighting the programming
And Jeanne showed her true colors when they mattered. She COULD'VE just gone and expose the island, take the compass and let the organization win. She didn't. She CHOSE to be good. Because she interacted with the Dikadians and saw that they were nothing like what the Organizations brainwashing painted them to be.