We were discussing something along those lines here at work earlier and I posted a little of what I think is happening in Tyler's MK ULTRA thread.
Take a good long look at the perpetrators of these mass shootings:
Loners, smart, spends a great deal of time online gaming or similar, possible (more than likely) personality disorder
They spend a great deal of time playing violent games, watching violent movies where there are no clear distintiion between right and wrong. The "good guy" has to shoot an innocent to get the terrorist or only gets a couple "points" taken off but racks up a huge score the more bodies. Little sense of fact/fiction or they become desensitized to the violence.
Then something sets them off and they turn violent in the real world. Lanza killed his mother, the VT shooter went after a girlfriend. They realize its "just like the game" so they follow the "storyline." It isn't real to them, just a "game." They also have been taught tactics and techniques, since movies and games are getting more realistic. Look at Call of Duty for PS3/XBox, it has a rifle that you have to reload when out, not just point away from the screen or press a singel button. The game reequires you to slice the pie and use cover and concealment to fight through the threat. Once they start getting resistance, either a weapon's malfunction, CCW bystander, or LEO, they do what happens in the game, die.
Is everybody that plays those games evil and going to kill someone, no. But you take a marginally stable individual, socially awkward, with a disconnection with reality and set them in front of the TV, movie, or game for hours at a time where that's all they see and then wonder why we have them go off the deap end.