One my wife’s sisters is like that too. We think that she is manic depressive. She only shows up during a rare family crisis to disrupt, intimidate, and destroy any family get together. Same thing; perceived injustices done to her going back to childhood and personal airing of grievances. She is in her mid 50s.
I’d say he needs to get to church and learn to forgive so he can let go of whatever’s poisoning him inside, real or imagined. But, he’s one of the most vocal churchgoers in my mom’s family. Hasn’t embraced some very basic tenets, obviously, and to his detriment.