People have always blamed the criminal actions of people on things they don't understand. Today, it's video games or metal music. In the 60s, people blamed popular music and hippie culture. In the 1920s they blamed immigrants and alcohol. In the 1500s, they blamed witchcraft.
I think it's because they don't realise that some people don't really need a reason to do bad things, and so they need something external to blame to preserve their view that it isn't society's fault, or the parents' fault, or the fault of the people who did it (although, since people who commit these specific sorts of crimes often have mental illnesses, I'm inclined to say that it's not entirely the perpetrators' fault in that regard; they could have been misdiagnosed or prescribed bad medication). It's easy for people to blame something that doesn't effect them personally, because it's a way of saying, "I'm not the one with the problem!" and pretending they're immune from similar situations occurring again.
I always like to point out to people the fact that the world was a lot more violent before video games were invented (and that the most violent parts of the world are places where video games aren't widespread). Video games are a hobby, and people who have hobbies are more likely to stay away from crime, especially when they give you a sense of accomplishment and a way of socialising with others - meeting like-minded gamers.
From personal experience, I can also say that most gamers I know in real life are pacifists, even though they play violent video games, and the majority of people I've personally met who have violent tendencies towards others have rarely if ever played video games - the reason they tend to behave violently is either because they were raised in a climate where they learned violence was an acceptable way of expressing themselves (and have acknowledged that in conversations with me) or because they had anger problems that stemmed from real-life experiences with other people. I think that, if they played video games, they would have a competitive outlet that could help them relieve stress.