Seriously mental, instable people in a crisis...
Another possibility is that it has relatively little to do with culture. Why do most people, included non-binary whoever lbtgqrstuvw self-identifyers, NOT go on a shooting rampage in schools, mosques, synagogues, bars.. while exposed to the same cultural stuff, being equally human on the biological level... living under similar circumstances, equally vulnerable to depression, psychosis, suicidal impulses, drug abuse whatnot...?
The exception affirms the general rule: most people from all possible backgrounds don't go there. The few that do...what can their expressed "motives" say about the act? Very little... I suspect however that often a
psychosis is at play as the key disrupter. Then add whatever helps further destabilize the person, like depression, resentment , hatred, etc. But without psychosis there is no serious there there, me thinks.
I find it sad, and even somewhat "mental" (tm) that Repubs and Dems can't resist the impuls to politicize everything, trying to score points in the culture war over innocent dead bodies and seriously disturbed, mentally deranged killers with a psychosis on steroids.