If you've ever seen a military or police demonstration on assembling a high-powered rifle, you realize that there are many parts to such a gun. There's the barrel, the scope -- well, I don't know all the proper names for the parts but there are many pieces, and it's impressive to see a trained person assemble the gun so quickly and accurately.
My point is, I believe that violent video games are like a trigger. It's a part of a fully assembled weapon. Without the other parts, a trigger is useless.
When a young person lives in a stable home, attends school, has friends and hobbies and activities, interacts with family, seems generally emotionally healthy, and plays video games, it seems pretty unlikely that playing a shooter video game along with lots of other kinds of video games will result in that person's becoming violent, heartless, and evil. The video games will not become a trigger on a dangerous weapon. They're just occasional diversion.
However, when the rest of the "weapon" parts are there, all that is needed is that trigger. If the young person lives in a home where he or she does not feel safe, if he or she is obsessed - to the exclusion of all else - with violence, if he or she is failing at school, if there has been a huge disappointment (rejection by a girlfriend or boyfriend, or being denied acceptance to a college, losing in a competition, or other significant loss), if there's been a death of a friend or family member, if the person is severely anti-social, if the person seems sad or anxious in an extreme manner, then I believe that those are all signals that the "weapon" of their destruction is being assembled, and adding in violent video games might just be the trigger that is needed to complete the assembly.
I believe that mother of the CT killer needed to recognize how many parts of this figurative "weapon" were already being assembled in her son's soul. For her to feed him with the assault weapons that she collected, and take him shooting those assault weapons for "sport and fun" is in my opinion the bottom-line blame of the tragic events. And if she was worried about him as she apparently was, violent video games should have been eradicated from their home.
I have a child with multiple medical and mental-health issues. Video games and computer games are her link to the outside world. And she's good at them. But although she plays some war games, there is a limit to the violence (no random gang violence games, or games that let you shoot innocent pedestrians, for example). And I pay attention to her. If she's feeling sad, or having a bad day with pain, or if she's feeling lonely, I won't let her just hide in her room with a video game. I'll play a fun game with her, a lighthearted game (which I always lose!) or a racing game. And yes, it's usually video games that are her distraction, due to her medical issues I just can't suggest a walk in the park or a day at the zoo, for example. Video games can be played just sitting with very little physical demand.
I have another child who has graduated from college, has a girlfriend and a million pals, goes out, works, has a nice apartment, and I wouldn't worry if he wanted to play a violent shooter game, because I know that it would be a brief break from a long work day. He got the chance to shoot an amazing rifle at a friend's ranch, but it was well supervised, and his friend's father is well-trained, and my son was telling me how, before they got to hold the weapon, his friend's father instructed them all on safety. It was way way out in the desert, and they had to wear eye and ear protection, and they were taught about the gun beforehand. None of them went out and bought a gun afterwards. It was understood this was an interesting experience, a once-in-a-lifetime experience. The other parts of the emotional "weapon" are not present in my healthy son's life.
However, with my other child, I am much more concerned. Due to her anxiety and depression, I would not let a gun enter our home, and I would not let her participate in an experience that involved danger or destruction. If she gets to do something fun, I'll make sure it's something like feed a baby giraffe or something, uplifting and creative and joyful. I have to be vigilant about her emotional state. I do believe that some of those emotional "weapon" parts are there: poor health, near-constant pain, loneliness due to medical issues, anxiety, depression, not being able to go out with friends, etc., and so I will not let down my guard about violence. We don't watch the more violent tv shows, and if she plays a war-type video game, I help her balance it out by playing a silly game like Fruit Ninja with her. Yes, it's tiring. I don't want to leave my bill-paying or scrapbooking or laundry to go drive a silly car around a crazy track on a video screen, but I'll do it. And she always ends up laughing at me because I somehow drove the stupid car backwards without knowing it.
Sorry, this is way more than you asked, but it's my everyday life. And I think about the violent video games a lot, and their potential influence. I think they are the final piece of a fully-assembled weapon of destruction, not the whole weapon itself.