http://news.bbc.co.uk/2/hi/europe/7936817.stm
[WS:] This seems to be a pattern that is not limited to the US society, and as such, it calls for an explanation.
My hypothesis is unprecedented glorification of virtual or symbolic violence in mass culture. Gory representations of acts of extreme violence are not only ubiquitous in popular culture, but they are also signifiers of coolness and power. Contrast that, for example, with representation of gory violence in medieval and Renaissance painting (cf. act of martyrdom) where inflicting violence was depicted as reprehensible, and suffering it - as noble. In today's popular culture, the reverse is true.
The reason for this is the market. Cultural messages and their producers compete for attention, and nothing grabs attention more than something that is shocking. The more shock, the more attention grabbing, and the greater the chance of market success.
In that cultural milieu, a few deranged individuals take this message too seriously and act it out. Given their access to lethal weapons and relatively lax social control of deviant behavior in democracies - they are very successful.
This hypothesis is not the same as "blaming the mass media." The media are merely a part of the market-produced mass culture - they are affected by popular tastes and demand just as much as they shape them. This is yet another example of damages that capitalism and its profit ueber alles mode of operation inflicts on the humanity.
Wojtek