But the kid was thoroughly trained in firearms safety and marksmanship.
Yep... It seems to me that MOST of the mass-shooters I can remember in my lifetime were fairly proficient in operating their weapons. I can't see that any more "safety" training would have made a difference. Most of them had serious mental deviations going on (some magnified by drugs) and, at some level, were obviously intent on killing people and causing mayhem.
Many political assassins, as another example, are very familiar with safe and proficient weapons handling.
It's not the point.
People generally don't absorb a balanced mind, basic morality, and stance on right vs. wrong from "gun class".