Well, I don't really care as it's not really interesting in watching individual cases.
I do believe that SOME very young people can get inspiration by violent movies/games, but it the source of the problem is not there. I bet that in 99% of such cases (not that there even are a hundred), you'll find some other major reason behind it. Abuse in childhood, bad parenting, bad friends/milieu, you name it. Normal people won't really be affected. If they were I wouldn't dare to walk the streets, as a large part of the population play these games.
So then we're left with the inspiration part. But people who are affected by this probably watch all sorts of weird movies they find on the web (which really can mess up your mind) and other such things. Of course I might be way off here, but so might you...impossible to know. Personally I think games like Manhunt are just ridiculous, but in games like GTA the violence actually play a more, let's say, "normal" part. You carry out mission for gangs and such - while in real life isn't something I condone - it's more normal. Making snuff movies isn't. I also believe that movies mess more with your mind than video games for the single fact that it looks way more real...
I could write lots more on the subject, but I guess this is boring reading so I'll stop here