Interestingly enough, there are quite a few factors that determine murder rates and crime statistics in a region. For example, it is possible to have a tangible decrease in an urban region by planting trees and other greenery along the sidewalks.
In general, though, it seems to me that isolated incidents of mass violence tend to be perpetrated by individuals who, psychologically, feel that the outward world has prevented them from achieving life-goals or goals that they feel they deserve. The interesting thing is that the pathology shifts with cultural norms.
The effect of this is that sociopaths tend to perform actions that society, paradoxically, expects of sociopaths. These are usually actions that mass media and the like glorify, such as terrorism and such. A good example of the effect was in Victorian Era England, where the societal pathology then, cannibalism, was acted out by sociopaths of that era.
In general, however, media and glorification of violence doesn't appear to cause these isolated incidents. They shape their form, but there is a higher correlation with other statistics like poverty and lack of social mobility than there is anything else.
In terms of gun violence in general, the sociopaths differ from random gun violence in that random gun violence is often perpetrated by individuals pushed past their breaking point and lashing out in stress. These tend to be one murder/homicide deals. Whereas the sociopaths tend to actively fulfill the "vendettas" that mass media thrusts onto them.
Bottom Line? Eh, it's a toss up.