Obviously, the guard was aware that the owner of the business was sitting in a chair to the right of the doorway. He's in view, until the intruder brandishes the gun. At that point, the owner backs away towards the wall, out of view. He also saw the woman on the floor to the right. Since he can no longer see the owner, he can't know if/when he might move into the line of fire.
The man with the handgun does not flee when he sees the guard with the rifle. He shoots. Then he ducks behind the opposite wall, hugging that wall just for a split second, with his gun still pointed in the direction of the guard and owner. Just a bit later, the security guard's shots pierce the wall in that spot. Just a split second too late, because the guy jumped back and headed for the front door. That's probably the safest shot he could get off, and it wasn't way off the mark, at all. In the confusion, he probably wasn't even sure how many attackers there were. All he knew was his people were on the right.
So guy is shooting at you, then moves to the side of a doorway with the gun (only thing still visible) still pointing at you about 3 feet away... who here isn't going to take a shot through the wall?
