I used to play a LOT of online shooter type games with interactive communication between players, and for a few years even owned and moderated multiple gaming servers. I can definitely attest to the fact that gaming can alter the mind, based on my experience with thousands of people playing these games. frequent players over time are remembered. and trust me when I say there were more than a handful of people I encountered that at first seemed completely normal when they first started playing these games, then over time these same players would become more and more violent and unpredictable in nature/personality, and threats amongst the players are the norm but some here and there would stand out. considering it's all anonymous, there's no proof of anything. but there definitely were a few people over time that I was greatly concerned about in regards to what they would do in reality if someone simply called them a bad name or worse. and again, I am referencing people who were NOT like this when they first started playing, but BECAME this way over time after playing more and more.
I would never FULLY blame a game for the actions of an individual, but I will definitely say it is my experience observing thousands of people playing games that these games do definitely play a part in this whole thing, either by encouraging the violent crime, or giving the criminal a specific act to attempt to accomplish, or even a dream to live up to.
heck, I've even heard people who were talentless in the online gun game say things like: "I may suck at this game, but when I come to your house with a real loaded gun, we'll see who is better".
the people that were actually good at the game typically were the good guys, and did not appear harmful in reality. it's the ones that sucked at the game no matter how hard they tried that I would develop a concern over. because they would know they were not good at the online version, and would get more and more angry about it and strive to do better and better. then every night they'd go to bed dreaming they could shoot more people more accurately in the game, and would wake up with the same idea but started to inch its way into a reality.
yes plenty of responsible people play these games and are never affected. but I am sure there are some that ARE affected in negative ways by these games.