If Brannom hadn't been shot to death, do you think he would have betrayed Emmerich? After all, Brannom was the one holding the gun and he had insisted that the diamonds be given to him (by the other crooks). I wouldn't be surprised if Brannom had taken off with those diamonds without sharing with Emmerich.
The opportunity was there. And, the motivation, given his "inferiority complex" and feeling he "should have been in the money long ago." However, Brannom may have felt some loyalty to Emmerich, knowing he was broke.
Generally there's no loyalty among thieves, which is why I think he would have betrayed his boss. Maybe he would have felt sorry for him and he might have tossed him a few diamonds before taking off with the rest.
Also, Emmerich said that he was broke, but he owned a lot of property. All he had to do was to sell that extra house. Of course he didn't want to, because he had Blondie living there, but couldn't he figure out that he would never get to have sex with her? Clearly she didn't want him that way and he wasn't going to rape her. He should have been smart, kicked her out, sold that house, and he'd have the money needed to pay off his debts. Stupid man.