To me the cause in incidents like this is a simple one. These people have distanced themselves from humanity, and humanity has rules. Whether it's caused by mental illness, drugs, bullying, the result is the same. Humans have no value because these people feel separated from it. Other humans become "things", much like any object setting on a table.
Most often there are warning signs, but when and who is the ultimate decision maker when a kid quits becoming disenfranchised, and becomes a threat? The state? A teacher with 40 kids in a classroom? A parent who's love for their child skews a decision that may be clearer for a stranger on the street? These are tough questions, and I'm not sure there will ever be a clear cut answer.
I know it may sound cliched, but hug your kids, listen to your kids, and teach them the value of life.
