To blame video games for causing violence, is the same as blaming the air you breath for causing violence. I could easily say "if it wasn't for the air around you keeping you alive, you wouldn't have been able to kill those people!"
Radical example I know, but do you see where it's going?
Any argument that says "x" causes violence is a load of bull.
i.e: guns cause violence, drugs cause violence, games cause violence, air causes violence~ etc.
At the end of the day, the responsibility is on the INDIVIDUAL, not on the objects around them. I could have a gun in my back pocket, smoke some weed, drink a beer, breathe some air, all while playing some Halo and I GUARANTEE you I won't go out "commit some violence."
All those studies done where X-Amount of people were tested and their results are also ***. Because again, if you tested One-million people who were just like me, what were your results be? 0 people committed violence.
This argument is almost as stupid as those girls who tried to sue McDonalds for making them fat...
It all comes down to people taking responsibility for their own actions. Sure there are those with mental illnesses, or younger kids who want to "replicate" what they see in the video games (which is why there is an ESRB rating on each video game). But if you are a grown adult 17+ and you allow yourself to be "influenced" by a video game to commit violence, you have issues.
The question you should be asking here is not "Do video games cause violence?"
The real question you should ask is
"Do you have the mental capacity to allow yourself to NOT be "influenced" by the video games you play?"