Things have happened in Faith's life that would completely screw up an individual. We know she came from a broken home, her mother died, and she was more than likely abused as a child. Not to mention neglected. And her repression of these events would lead to a lot of built-up anger and agression. I think that one day she would've snapped, whether or not she was Chosen. She's a terribly broken girl. It's hard to say what she would've done, but killing people probably isn't too far off the mark. In a way, being the Slayer saved Faith as much as it proved her downfall: it gave her a purpose, and likeminded people, however loyal or not so, to share this experience with. And without that feeling of purpose, I think Faith would've gone off the deep end, and probably? She would've commited suicide. All that loneliness takes its toll on a person.