There is a LOT going on here legally under very old laws that could be outdated.
Citizen's arrest laws are always a bit weird. There are so many 'what-ifs" that on one day you agree with citizen's arrest ability and on another day you think they're the dumbest thing in the world.
Then you combine citizen's arrest laws with open carry laws. So maybe guns are involved.
Then you have a situation where a fight OVER a gun breaks out. Self defense? Not self defense? Accidental gun discharge during the scuffle? Malice/fear/emotional state?
This is a case where bad laws are combined with very morally suspect people (likely on both sides), throw in different races, throw in a gun, throw in video and you get almost a no-win situation.
I really can't think of a good outcome here. The guy is already dead. I think these hillbilly ex-law enforcement people went WAY overboard and took the law into their own hands but there isn't really a CHARGE that looks like it sticks.
I think you go for Manslaughter.... 3-5 years in prison. I know that seems like not a fair trade, but I'm not convinced the victim was completely innocent of wrongdoing. And once you establish that, it opens a whole can of legal worms for these guys to get off on.