There's probably a correlation between the fact that the US is the only nation with a mass shooting epidemic and the fact that Americans are the most aggressively propagandized population on earth.

If you took any armed population and psychologically pummelled them from birth with narratives about how mass military slaughter is fine while turning them into underpaid, alienated gear-turners and giving them an artificial culture mass-produced in Los Angeles, you'd probably see some mass shootings.

There's only so far you can warp the human psyche before it snaps. Bash hundreds of millions of people in the brain their entire lives with indoctrination programs telling them madness is sanity and sanity is madness, and eventually a few of them are going to wind up mass murderers.