After Sanctuary, Faith became a pacifist. She swore never to raise a finger against another living thing.
Everybody around her supported her new ethical stance. They told her that the end never justifies the means, and that it's always wrong to get your hands dirty, no matter what the reason.
These lessons fitted well into Faith's over-inflated sense of self-importance., She'd never dream of sacrificing herself - literally or metaphorically - to serve some greater good.
After all, it's not that she thought that putting yourself through suffering and degradation was a valid way of atoning for your past crimes.
No, the lesson Faith took from her time in LA was that it was best to withdraw from the world, and not risk harming her new ethical development by getting involved in anything that might be morally dubious.
Indeed, she was now contemplative and withdrawn. Never again would she rush into a fight against Evil without weighing all the options, and considering whether she was doing the right thing, or whether the evil she was told to fight might actually have justification on its side.
And above all, the thought of raising even a finger to harm another human would never, ever cross her mind again. She'd flinch at the very thought.
And why not? Because her own purity and shining moral perfection are all that matters to her. Some might think her selfish, but she didn't go through all the pain of redemption to throw it all away by getting involved in other people's problems.