Too complex for black and white
Computer games, music, youtube videos, Tony Blair, etc, anything can contribute to a persons instability, then give them access to a tool of violence and they have a better chance of making their "scream" heard. You can never point you finger at one thing and say "There! That thing there made little Johnny go to the gun cabinet.", it's simply too complex. Given today's complex and pressured lifestyles, especially among teenagers and young people with the myriad of social pressures in their lives, any combination so things can trip someone and make them find a solution to their frustration. We pray it never happens to our own children but every single human being on this planet has it inside them to do exactly that poor wretch did yesterday, it just takes the right combinaiton of triggers and the blue touchpaper is lit.