I have a personality disorder, and have done a lot of dickish things in the past due to it. I lost boyfriends and friends over how I acted and they were right to ditch me. I was a fucking nightmare.
So whether or not the person has a PD or not is sort of beside the point. If someone's behaviour is hurting you, you don't have to put up with it. That person may have a REASON - but it is not an excuse.
It's hard really. On the one hand, in the midst of a bad episode, I really couldn't control myself. I used to say the most horrible things and as I was saying them, I said them in order to hurt people. But I felt like nothing I said would ever hurt anyone, because I was so insignificant. And I really had no other way of coping with how I felt inside, I felt like an animal. So I do think sometimes it isn't exactly the person's fault.
But I don't think it should be treated as an excuse. "Oh, she has BPD, she can't help it." Well, maybe not, but then I don't think you should be in a relationship with someone whose behaviour is consistently disrupting your life and making you feel bad. The kinder thing to do would be to leave that person. It sucks for them if they are suffering, but you have to look after yourself first.
Chippednailvarnish - "only directs unreasonable and unacceptable behaviour towards one person, then its not a personality disorder" - I actually disagree with that, because my personality disorder really only comes out in romantic relationships. I find it hard to be close to people in that way and I panic. In fact, for borderline, I think it's actually relatively common for people to appear pretty normal outwardly and then to unravel at home.
Personality disorders are complicated things and I would give anything to not have this. Anything.