Tailgator: There has been research into the effect of reaction times on the decisions to start and stop firing, and it is quite possible for an assailant to change orientation (in this instance by turning away) in the amount of time that a decision is made and acted upon by the brain, nerves, and muscles. IOW, it is quite possible that a shooter can start the process of pulling the trigger while an assailant is facing him/her and still shoot the BG in the back if he turns during the trigger pull.
IIRC, the info was from Force Institute. I'll post a link or two later if I can find it.
In reference to your comment mentioning the BG being shot in the back, and the decision time to stop firing, in a book I have by Michael Martin on Concealed Carry it quotes the Tempe Study, regarding an attacker being shot in the back and says that 2/10s of a second the BG could complete that movement of turning and the defender might require 5/10 of a second to stop shooting. I figure that might have been what actually happened here.
BG saw the gun, started turning to run, got shot in the back before the
15 year old boy could stop his reaction to the BG.