Yes and no. He could have shot, but by the time he had the gun under control, it would have been a fleeing shot, and the video tape would likely put him in prison. No shots in the back allowed. (no imminent threat To life.) At that point, cops would call it murder.
True enough, hence the word "likely" not certainly. You can not even say with certainty, that being shot under those circumstances would be fatal. (If the clerk could hit him at all.....)
no, the cashier did not have the right to shoot at him, at that point, even if the punk was still facing him. the thief was showing no other weapon, making no violent move, so firing would have put the cashier in prison, most likely, and certainly gotten him sued