I have a friend that I don't think I want to be friends with anymore...but I wonder whether I'm being unfair.

Here is the situation in a nutshell: we met in school 5 years ago. Have always had a tumultuous friendship, in part due to my mood issues and her own mental health issues (unclear what they are exactly - sometimes she says OCD but I think she has a personality disorder). In the past we went an entire year without talking after a big fight. We had another big fight in October and I haven't spoken to her since. I just received a second email from her apologizing and wishing to make amends, but I just don't know. I'm an incredibly forgiving person and I do not hold grudges, but I worry that she may actually be a bad person. She plays the victim constantly and always thinks everyone is against her. Then she'll get totally obsessed with how she has been 'wronged' and lash out in ways that are totally inappropriate (for example, posting private personal emails on facebook). It seems she turns on people with the flip of a switch - very jekyll and hyde. On top of that, she can be kind of nasty to people who aren't on 'her side' (like making fun of how much weight someone has gained if she thinks they treated her badly).

I feel conflicted because on the one hand she is not the type of person I want as a friend (even if she is nice enough to me) - and I don't think I could ever really trust her again. However, I wonder if her behavior is due to some kind of illness or disorder, in which case it seems unfair for me to abandon her. I mean, we did use to be extremely close...