No one seems to be mentioning that the people who do stuff from video games are just insane. They probably would have done the same thing on their own, or imitated some other thing. There was the story about that kid who jacked a bunch of cars and shot someone just to see if it was as easy as in GTA, and it was reported that he had "no sign of any mental disorder or disability." Excuse me if I'm wrong, but isn't imitating, especially a bad and stupid thing, a video game, movie, or TV show a sign of a mental disorder or disability?
I'm sure at least some people let their anger out in video games. Dante put the peopel he didn't like in Hell, the Columbine kids shot up the place(because they were crazy, not because of DOOM or Marlyn Manson), others write fanfic where Harry and Malfoy zap the jerk before they have sex, and I'm sure some people imagine the enemies in the game, or even mod on, as Chet, Trevor, Chad and the boys, and gleefully blow them up. I have now linked video games to decreased aggression. We should put more ads up in place of anti-depressants, send Halo to Juvie Hall, and band Waldo Emerson from school for leading to increased violence by not decreasing it like video games. What about the people or have nothing to do but hang out on the streets? I'll bet that someone was only going out to vandalize because he was bored, and now that he had an Xbox stayed home instead. 100% of the test subject showed 100% decrease in crime rate and vandalism.
To answer the OP's question, they could be right, but we would have noticed it before. If they are right it's change is very minimal.
Video games have even more warning on them than T.V. and movies, yet there is no big warnings on theme, and those are almost the same. Yes, video games are a risk factor. Someone eating a hot dog is not going to give them a heart attack. Having a medical thing, not exercising, and eating lots of fatty foods will give them a much larger chance of one, same with video games. As much as there are arguments about cigarettes, but they do at least have chemicals and whatnot that are just bad for you. There's a lot difference between harmful chemicals and seeing/virtually doing something cool.
Remember, if nothing else kills you in your life, carrots will.