The kill-to-injury ratiơ suggested to me immediately that the shooter was a long time video game player. Those games train the player to be accurate hip shooters. A decade or so ago there was a spate of very young store robbers who shot clerks in the forehead and, after capture, protestations that, "I didn't mean to shoot!" because the killer had been video trained to shoot at the slightest movement. It was unthinking reaction.
Ive read a few reports that says he was an avid gamer. I wouldnt be surprised that he was into FPSGs. I believe that even the military has used some of these types of games for training purposes.
In any case, it wouldnt take any amount of great skill to walk up to cowering little children and shoot them in the head, which explains why the death toll was so high. FWIW, he shot his own mother in the face, probably did the same to those poor children.
I’ve been playing video games of all types since the late 70âs and your theory is misguided.