I don’t think it’s astonishing that he killed six adults and twenty children in a school. They were unarmed and probably froze. Shooting a room full of kindergarten children doesn’t require any notable skikl.
You're quite wrong, actually. In almost every slaughter (and there have been grade school slayings before), there's about a 2-to-1 ratio of injured-to-killed. (Columbine had 25 injured and 13 killed.) Even highly-trained LEO's barely crack 10-15% hits in a real-world shooting. Even if the kids were frozen in fear, hitting and killing with such... precision... is remarkable.
Either he held the gun to the head of every kid (and even a 6 year old won't sit still if he has just watched 13 classmates get thusly executed), or he was firing a LOT (which means he carried many, many clips, since he was using handguns. Since reports refer to a "pop-pop-pop" sound, I'd say that he was NOT walking up to each victim. Hence, he was shooting from a few feet away.
Further, shooting round-after-round GREATLY decreases the accuracy of your aim. Go to a gun range sometime. Try it. Then try having a friend stand behind you and simply COUNT, calmly, "one, two three"... just THAT tiny added pressure will make otherwise solid amateur shooters start trailing off by several inches. This 20 yr old unpracticed and untrained amateur had screaming kids, knew that police would be there soon, most likely SOME moving targets, AND he was shooting at a few feet away, AND was not taking time between shots to regain his aim point... and STILL he had more than 20 TIMES better of a kill ratio than almost any other similar massacre in modern history.
THAT is worth talking about.