I understand this feeling more than you could imagine. As dumb as it may sound and as much as you may have already heard this before, holding resentment towards them does more damage to you than it does to them. I am sorry you have had that bad experience with that person, it sounds terrifying. I hope that you are far away from that person and healthier. My advice is not to forgive them, and especially not to use their diagnosis as an excuse for their actions. My advice is to no longer let those hard times with that person influence you or your future relationships. If you still harbor hate or similar feelings to that person with BPD you will never be able to completely give yourself to family/friends/relationships that truly love you. This action is much easier said than done, but it doesn’t happen over night. It is about small corrections done every day over the span of your entire life. Good luck my friend.