Hi RunMom,
I'd like welcome you to bpdfamily. I'm sorry that you're going through a difficult time. No worries, this is place is here for members like you to vent and to get help with someone that has a pwBPD in their lives.
I can relate with a child being split black over the other kids, it's heart breaking. This is just my theory but the T may of recognized that there was drama triangle at play and the reason why he said to stay out of it is so that the triangle would collapse and not perpetuate drama.
BPD is a mental illness that has the most coexisting mental illness attached to it and experts are not sure why, but BiP is a mental disorder that is comorbid with BPD and BiP is often confused with BPD. It can be difficult to identify all of the disorders that someone has, you want someone experienced with BPD.
If he does have BPD, it's a persecution complex, so the person really believes that their circumstances are caused externally, not from them but from others. PwBPD blame others and the world for their problems, don't take the blame for his actions / behaviors.
Now, I would be scared if my SO had the capicity to become violent, is your oldest out of the house? Have you called a women's shelter, they have trained staff that can help you with an exit plan, do you have an exit plan? I just to let you know that we're volunteers here and cannot help with an exit plan.