Oh yes, creek justice, mountain justice, is a thing. Happened in my inlaw's family just a few years back. Their kin will now spend the rest of their lives in prison. Ripped the family apart. If it comes out, that the Rs have done something, in the eyes of the assailant(s), as unforgivable (hurt a child, possibly sold drugs to another's family member, and that family member died, or something similar) then yes, I'd go with mountain justice, and have not ruled that out. However, no, they'd not have to be psychopaths. If it comes out, that the Rs have done something, in the eyes of the assailant(s), as unforgivable
However, we've not heard that the Rs have done anything like that. For those who believe this to be about custody, that's not what I'd call a reason for creek justice. To murder eight people, only because you don't want to go in front of a court, and hammer out custody agreement, or because you want to run off to another state and take your child/grandchild, that's purely about self, and, would be closer to psychopathy. IANADr, but psychopaths have a grand sense of self-worth, no remorse, no care for others feelings. They want what they want and go after it, not caring about the feelings, or fall-out, they leave behind.
If this is about hard drugs, then that's a risk of getting into illegal drug sales, and that culture. You'll rub elbows with some very sketchy folks. Folks that look, and talk, one way, but are something completely different underneath. They're chameleons. Harsh, but true. Are they psychopaths? Very well could be. Psychopaths are in every walk of life. They are cut throat and do end up in leadership positions, quite often. They don't always kill, either, but if they have to, they don't feel badly about doing so. I do feel that this could have had something to do with drugs (not necessarily weed, though).