Blame several factors.

Social media. Now there is instant exchange of unfiltered, often dangerous ideas.

Drugs. Families decimated, kids brought up in the aftermath.

Religion. Either none, or extreme religion; used to be a "normal" religion helped instill morality without zeal.

Violent video games. Life imitates art.

Divisive culture / politics. Lost souls searching for meaning grab onto crazy notions and tolerate nothing else.

Media exposure. The national publicity of a slaughter implants the notion in the noggin of losers who want to die anyway; they then figure "If I'm gonna go, I want people to know about it...they never cared before."

Per Pogo: "We have met the enemy, and it is us."