Let's try and look at this from the viewpoint of our BG.
His simple spur of the moment robbery, has turned into something that he didn't expect and he is behind the curve big time.
His options are somewhere between zero and none, he is in fight or flight mode, but he can't run with the police outside.
Now we have a panicked clientèle within the restaurant and this is just playing on his nerves big time and he might be going into all out panic mode.
So, the gun is to the head of your significant other and we have a scared adrenaline hyped BG and no idea how many sleepers might be in the crowd (always consider this) and you are facing him or at the least in his peripheral vision and you think you have a shot, you move , he is startled turns towards you and using both hands to turn his hostage and because the motion cause the muscles in his hand to contract and BANG, the gun goes off killing your significant other.