Hey there. You might find it helpful to read something like "I hate you don't leave me" or maybe even Marsha Linehan's book on how to treat BPD. Both of those offer really good insights into the rationale for borderline behaviours from a fairly non-judgemental stance. It can indeed be hard to see what could motivate cruel and abusive behaviour.

I'm fairly sure that there are support boards out there for family members and friends of people who have BPD. I've never looked to any personally. I'm not sure how useful they are and how much they disintegrate to mudslinging and venting.

I know that having a family member / friend with BPD can be very challenging indeed. Really very challenging. Can rip the family apart. Can split the family into the group that tolerate the person and those who have had enough and simply can't cope with the person anymore.

Oftentimes the family members / friends of someoen with BPD find benefit in councelling themselves so that they can come to understand some of the rationalle for the borderline behaviours. Also to process some of the feelings of helplessness (that you can't fix the person) and anger (in response to their apparant cruelty and disregard of others) and shame (that they feel angry toward a person that they love) etc etc. People with borderline personality disorder are emotionally intense individuals and as such they tend to elicit intense emotional responses from others. It can feel very emotionally draining and sometimes the person simply exhausts our capacity to deal with them...

I'd reccomend that you have a chat to someone (a therapist) about what has happened. Death of a family member can be very hard indeed. The circumstances around the death make it even harder. The mixed feelings you have about the person (good and bad memories) make it harder still. The incomprehensibility of their actions... Still harder.

Of course you might find the support that you need off boards... But please don't be afraid to get some therapy yourself to help you deal with this stuff. Would be really very hard for anyone...