Have you tried shooting fish in a barrel?
More seriously, the excess of 90% is very impressive; unrealistically so.
Trained soldiers rarely get that high an accuracy in combat; and if you're thinking that CQB is fish in a barrel, you've never heard how the military had to adopt new tactics for urban warfare. Yes, when no one's shooting back you have less to worry about than a soldier clearing a building; but the sheer number of people moving/hiding combined with the panic of flight or flight instincts makes a hit-rate of 90% pretty damn impressive.
Perhaps not enough to on its own be bizarre, but combined with the other facts of the incident (drill-protocols followed, the lack of mandatory hazmat-cleanup paperwork, and the school being bulldozed/rebuilt) makes it stand out all the more.
Urban warfare? You’ve got to be joking. These weren’t guerilla soldiers he was fighting, it was small children cowering in fear that he was executing. You’re really grasping for straws here to try and paint this as anything other than a tragic attack by a crazy person.