I think the most important thing here is whether or not the person consuming the violent media has fantasies of violence. Purely anecdotal, but in my own life, I'm convinced certain types of media have pushed me over the edge from fantasy for reality. For example, ever since I was a young child, I found smoking women to be sexy. I fantasized about it in my adolescent years. Then I discovered smoking fetish pornography and that really got me going and I started to think maybe I should actually date smokers. Then I did and I started to think it might not be to badd to try smoking myself.
I doubt I would have made it this far down my current road had I never taken that initial jump from purely in my mind to consuming smoking fetish media. In other words, I think the smoking fetish media helped me take a harmless fantasy and turn it a ruinous reality. I wonder if the same could happen for other people with other (not even necessarily sexual) fantasies.