Take a kid, let him grow up with distorted values, get him addicted to vicious video games, leave machine guns around the house, have his parents separate and remarry. Let him know he is a runty, nerdy unloved individual with no chance of a satisfactory job. Give him a part time job at McDonalds. Give him no treatment when he shows signs of mental instability. Make him ashamed that he jerks off.
Bully him at school, call him a creep. Give him no chance to excel himself .Call him baby-face and cretin.
Give no adequate explanation to his worries that the world is coming to an end, of the financial worsening in the United States and the daily bombing the U.S. invasion forces are enacting on innocent people including children with exploding anti-personnel bombs , killing and maiming thousands.
In other words treat this young twenty year old like a crazy no-good bastard and one day he might go out and murder twenty nine people including twenty children aged 5-7 years.
Yes blame this animal but don't forget to blame the evil society he lived in too.