The late Elliot Rodger was a monster. I know partly what had produced him: feelings of humiliation, which is almost always followed by revenge. Hubris followed by Nemesis. It's been noticed for thousands of years. It's not like I figured it out on my own.
Then I also found out he never played, and that's associated with murder, too. Such people get no pleasure out of life, so they take the pleasure out of other's lives by taking their lives away. Envy and hate.
So if you really want to screw up someone, humiliate them and take all the fun and pleasure out of their lives.
Elliot never showed any signs of normalcy until a grown man got some girls to tickle him.
Play is also associated with creativity, with advancing culture, with discovering, with inventing. Actually, play is those things.
Men are overwhelmingly the ones who do the inventing, discovering creating. So, the more boys are prevented from playing (which requires trust and security), the more they are humiliated and made to feel guilty, the less creating and discovering is going to happen, and the more hostility they're going to show.
And the more civilization goes backward. It's already pretty degraded as it is.