In order to do something, you have to be able to imagine doing it. What constant exposure to fantasy violence, and real violence, through modern media - TV, video games, films etc - does is it makes it easier for us to imagine ourselves doing appallingly violent things.

It seems to me, it has to. And for some people that makes all the difference, and when something triggers them they move into killer mode, as if they were in some shoot-em-up game.

Ready availability of guns feeds in to that, and some of the characteristics of schools (and workplaces for that matter), especially in some cultures - cliquishness, bullying, social competitiveness.

Put them all together and it's strange it doesn't happen a lot more often.