The Buffyverse doesn't accept that kind of thinking. Its view has always been that there is ipso facto always hope for an individual to change for the better (and for the worse). There is free will versus determinism, hope versus inevitability, and I do find that wonderfully subversive. The basic theme that is so very visible in the entire Faith arc, from Buffy to Angel (and back to Buffy, although I haven't seen that yet) is not only consistent, but it grows in complexity throughout. From the first, it was a given amongst the Scoobies that they would try to help Faith when she accidentally killed the Mayor's assistant. It was definitely accepted that the act was wrong and she needed to deal with the consequences of it, but it was never just thrown out there that she should be tossed in jail and have the key thrown away as a part of some "tough on crime, three strikes you're out no matter what" policy that you'd see on Cops or a lot of dramatic police shows.
Of course, the tragedy is that Faith believed that approach, herself. She decided that because of that irreversible act, she was forever tainted and might as well go deeper and deeper into darkness, even while the others were trying to save her. But it was shown that her view on that was wrong. Just as Angel (and later, Faith felt this as well) was wrong to think that his only salvation would be in suicide. Both felt at these points that they were trapped by their pasts and could only continue to be evil and nothing else.
The Buffyverse disagrees. Angel's epiphany was that there was no greater scheme of good and evil to be accountable to, that he wanted to do good because it was the right thing to do, not because he wanted to be saved. A very individualistic approach, but without the idea that this freedom from an overarching moral censor gave him the freedom from morality entirely. That's the argument of organized religion, that mankind needs to be reined in with pronouncements from above in order to do good, and that atheists and nonbelievers are by default immoral, hedonistic, epicurean, you name it. That there's no honor among the free-willed. And Angel and Buffy go against that. I guess they follow the philosophy that humans are basically good at the core, even as they acknowledge the potential for evil and the temptations that are always around to lead you astray. The thing is, despite the sometimes religious sound to the words used like redemption, good & evil, etc., the message isn't a religious one. Here are 2 shows that put forth a humanistic, moral universe. How often do you see that? In season 4 of Angel, when Faith comes out of prison to save Angel and, even though she's made great strides forward in her search for redemption, she's still terrified of losing control and making a mistake that will negate everything she's done. It's made clear to her that that could happen, but if it did, it would not be the end. A lot of talk is made in Buffy 6 that there are some acts someone can perform from which they cannot come back. If Willow kills Warren, there's no going back. In Angel 4 it's expanded to say that yes, there's no going back from the evil acts you've done, BUT there is going forward. You're not irredeemably stuck in the same place; you're not on an irreversible path to darkness, no matter how dark the act you do. What matters is how you behave after the act.
That idea seems incredibly mature and applicable to more than just the big good/evil issues. It applies to all issues in a human's life. You can't change the past, you can't ever become what you once were, every choice you make changes you and has consequences that you are responsible for, but you are not bound by your past actions and you can still act differently in the future. And what matters is not ultimately the balance sheet of your life, how much good versus how much evil over the course of it, but what you are doing in the present, every day. Trying to do your best with what you've got and never giving up because it is hard or because you've already screwed up so bad, what's the point? Every day still matters and every day you make the choice of how you will act.