I rewatched that one not long ago. Good episode and I felt for Faith there. But still, what followed next from her I felt was absolutely unwarranted. And I don't think many 17-yr old go on murdering and torturing rampage after learning that their best friend happens to have a boyfriend. It's just so out of proportion - her reaction to the perceived slights and injustices. So out of proportion that I just couldn't care about those injustices anymore: they were tiny comparing to the awful things she did in return.
So any sympathy I had evaporated after she raped Xander, and after that it just kept getting worse. It's a similar situation when a sadistic serial killer brings up his unhappy childhood and stuff, and justifies what he's done with it. I won't have a shred of sympathy for such a person until they realized that there's *no* justification or excuse for what they've done.
So until Faith got there I didn't care for her.