Where do you draw boundaries with a serious mentally ill person?

This has been weighing on my mind a lot lately. I have posted about abuse I had from my mother since childhood here before, but this query bases around if you have ever known a sociopath yourself, or if not, where would you draw the line at their illness, or them just being mean or cruel? A short history is my mom has OCD, is manic, borderline personality and also borderline sociopath- she rages, curses, hits, screams, threatens and has even threatened my life once and her own. She is obscenely obnoxious and excessively clean- but when she has a RARE moment of 'how did such and such go?' you can see a tiny TINY spark of humanity inside. It is so rare but yet she shows this to her mother, and strangers, often. A mask , and then tells me people 'love her'... so when she is being kind and humorous with THEM, but never with me , for years, I know she is capable of kindness in 'some' way. She then says the most cruel and horrible things to me about myself, and I don't see it as illness at that point, I see it as mean or cruel. (b/c she was not that way with them..) The thing is I Know she is ill ... the ocd, the excessive repeating herself, the becoming enraged over even a cup knocked over- where would YOU draw the line though between an illness and being downright vindictive, and would you completely cut yourself off from such person? (yes even if a parent)