Nurturing a culture based on violence and litterally exporting said violence since decades... Nurturing the drug traffic worldwide with all power available (see Afghanistan for heroin and Colombia for cocaine as illustrations).
Not just that but nurturing that differences be settled by violence with one war after another, mass media showing how mass shooters get attention and they keep a high score list, and both parents or guardians too busy to raise their kids so the kids run in the streets and learn from gangs and criminals shown on the news as getting as famous as heroes. We are showing the worst of humanity and rewarding it with attention and fame.
Europe: 53 dead, 169 injured U.S.: 392 dead, 1,502 injured
See https://www.vice.com/en_us/article/4wb9p3/mass-shootings-in-... and https://www.vice.com/en_us/article/xdwpwk/mass-shootings-in-...