It should be pretty clear this boy was either very disturbed or had not been taught most of the stuff he needed to know (I really don't want to blame the parents at all. Maybe it was his friends group or something?) Like myself for a form of example, I could only kill if my life or the life of someone I cared enough about seriously depended on it, thanks to my life-style, how I was taught by parents and the people I hung around with (we were a very pcifistic lot). Take away something I like and, yes, I will go a bit nutty, but nowhere near this.
And to repeat what most have said: Videogames will get the blame on this one.
This is just truly disheartening and sad.