I spotted this article via Twitter and it's application is general to US society, not just these murders:https://warisboring.com/what-on-kill...0f1#.6hssdabyf

It starts with:
Dave Grossman’s 1996 book On Killing is a landmark and studied account of how — and why — human beings have inhibitions toward killing others, and how the U.S. military turned its soldiers into far more lethal killers with intense conditioning following World War II.In an updated edition, he warns that these same psychological inhibitions are eroding within American society, but with fewer safeguards, allowing sociopathic tendencies to arise and enable mass violence.


(Later the author asks) But the question we need to ask is, What makes today’s children bring those guns to school when their parents did not? And the answer to that question may be that the important ingredient, the vital, new, different ingredient in killing in modern combat and in killing in modern American society, is the systematic process of defeating the normal individual’s age-old, psychological inhibition against violent, harmful activity toward one’s own species. Are we taking the safety catch off of a nation, just as surely and easily as we would take the safety catch off of a gun, and with the same results?
A UK academic psychologist offers a viewpoint:https://theconversation.com/can-we-predict-who-will-become-mass-shooters-60969?