Being shot "in the back" that you keep bringing up is also a misnomer, as it is phrased that way to imply he was shot with his back toward KR and no longer a threat. It was in the back because of the angle, and the defense concedes he was technically no longer a threat after the first shot.
If Aroldis Chapman threw a fast ball at the same time as KR's first shot, the ball would just be hitting the catcher's mitt as he's firing the 3rd and 4th kill shots.