And your numbers are off. 3800 FPS would be the 22-250 range, and there are much more powerful rounds available, even on the AR platform.
A 22 LR will kill rabbits and humans, but you're not going to take down an elk or a moose with one.
But the question you should be asking is why did this person want to kill as many innocent children as possible, because there are other means to do so that cannot be easily controlled.