This is an interesting incident. I find it interesting that the pawn guy allows the guy to advance on him while at gun point, then physically tries to stop the BG when the BG grabs the gun. Then shoots the guy as he's running away. I'm not trying to second guess his actions, but trying to figure out what the tipping point was for the pawn guy. It's difficult to tell from the video what the BG is actually doing when he gets shot.
To me, the zenith of the threat would be when the BG advanced and advanced again to grab the gun. But I could only imagine that the situation in real time one might not process all the factors and changing gears from detaining someone to using lethal force probably wouldn't come easily.