I think generally you are correct. However, one could argue a perfect storm of factors that can lead to a Columbine massacre.
1st - A broken home where either the parents are abusive (physical or emotiona) or absent from the lives of their children. In such conditions, they'll lack not just nurturing but guidance they need to cope and surivive teen years.
2nd - poor to non-existing coping mechanism which can lead to distructive behavior such as bullying, self-harm, or suicide. Children with absent parents are 10x more likely to commit crime than those who have.
3rd - In such troubles, often traumatic, entertainmnent to drugs and alcohol are often the go-to means to cope with their problems
4th - No parental guidance, means no control over the viewership of theri children. Children who are oppressed by bullies for example, would watch violent to sadistic movies like any other, but more then the rest would likely dream of using torture methods to machineguns to reap bloody vengeance upon their enemies.
Not all would of course, but we all dream of revenge, right?
5th - Operant and Classical Conditioning. This is the most frightening of all. According to Lt. Col. Dave Grossman, and others like him, the human mind can be conditioned to be desensitized to what normally is abhorrant to us. For example, Pavlov trained a dog to do certain tasks and if the dog had done what is desired, s/he is rewarded with a treat. Thus the dog
In another example, in the military, the recruits were trained to be desensitized to killing by 1st making the notion of killing, desirable through cadences or chants. 2nd, to shoot at realisitc human targets. The end result was that before, it is not uncommon for Soldiers to fire at nothing, or not fire at all; but after, the kills went up by 50%. Improved training resulted in 75% kills, etc.
6th - When kids, un-supervised or advised, watch movies that makes gory kills and torture a thrill,, or games that rewards a score for every kill to r&pe, then what you have are kids potentially desensitized to killing and others.
7 - Now that still doesn't mean that everyone will go out and act the movies or games, because again, killing does not come naturally to human beings. there;s this psychological thresh hold that prevents us from acting out in such ways.
However, with kids lacking parental love & the severity of their environment can stress the troubled teen finally override the threshold.