So, this may have to do with a 'business arrangement' of some kind. Interesting.
The type of person Joe seems to be, would not exactly go along with this way of thinking, UNLESS he didn't know how or what he was involved in. This would mean that someone else was involved. Someone else may have been the catalyst.
It was obviously not a crime of robbery, although they may have gained financially. Joe either had evidence of some kind that he didn't know he had acquired, or there was something he knew about, had no intention of revealing, but that someone doubted he would keep secret.
I still find his girlfriends behavior strange, in some way, but can't figure out why. Did she ever work at other bars/lounges? Anywhere where any other victims in OK lived or frequented?
Did Joe keep guns in his safe? Could one have been used in an additional crime, that Joe knew nothing about, but the killer did?
Anyway, thinking a lot about Joes and his family, praying for closure!!