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.
I’ve read a few reports that says he was an avid gamer. I wouldn’t be surprised that he was into FPSG’s. I believe that even the military has used some of these types of games for training purposes.
In any case, it wouldn’t 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.
Sorry... as a long-time video gamer (who held a world record for a few weeks once!), and a part-time shooter at the local gun range, I can tell you in VERY certain terms, video game talent has NOTHING to do with shooting accuracy... particularly after the first shot. Recoil, noise, pressure, etc are all INCREDIBLY impactful on your accuracy, and you get NONE of those at home on your couch. Then there's the hand-control. Squeezing instead of pulling... too much grip... actual FATIGUE (shooting for more than 5 min with a 45 or 9mm will tire you out a bit! 20-hour gaming marathons never prepared me for that!).
Add in autism or Asperger's, and ZERO firearms training... this was no expert, and yet his kill ratio was FAR higher than almost any other similar massacre in modern times. (Columbine has 25-to-13). This untrained kid's ratio of 3-to-26 is STUNNING, and needs a deeper look.