Even if the gf wasn't involved, she could have made an off-handed remark to somebody which set the chain of events in motion. Possibly about the money in the safe.
However, with the money left in the safe and on the body, I am leaning away from robbery as the motive. I really wonder if he didn't stumble upon something while riding his horses. He would have to be able to put a name with the illegal activity. Maybe this person came to see him before the bar opened and Joe let him in to talk. This person assaulted him. They didn't shoot him in the bar because a gunshot would draw attention so they took him out to the strip pit.
However, with the money left in the safe and on the body, I am leaning away from robbery as the motive. I really wonder if he didn't stumble upon something while riding his horses. He would have to be able to put a name with the illegal activity. Maybe this person came to see him before the bar opened and Joe let him in to talk. This person assaulted him. They didn't shoot him in the bar because a gunshot would draw attention so they took him out to the strip pit.