I think that there are two issues here, one is "cause" and the other is "contribution". The front page of The Sun today implies in my reading of it that Call of Duty is the cause of this mans act of violence and I think that is poor, oversimplified reporting on what is a very complex issue. In that case I think the media are scapegoating video games which are an easy target, also Call of Duty is a massive success and certain aspects of the British press love beating up anything or anyone that is a massive success.
The next question is: do videogames contribute to violence. That is a question I don't know the answer to. The causes of violence are multi-faceted: poverty, family relationships, other personal relationships, alcohol, drug addiction, feeling alienated, family breakdown and possibly certain psychological or emotional issues. There are also factors that are possibly to do with how masculinity is socially constructed: for example it is less acceptable for men to cry or express emotions. If men cry or talk about feeling humiliated or a failure they may feel less of a man for doing so. Of course the other contributing factor is the easy access people have to assault rifles in the united states.
So while, in certain cases, playing video games may be a contributing factor in the overall picture (not a cause) I believe that the thing that needs to be done in the USA to stop these events happening is tighter gun control.
In other countries of the world (such as the UK) violence from men towards both women and other men is a big problem. I believe the solution to these problems doesn't lie in banning video games. I believe the solution lies in encouraging males from an early age to express themselves emotionally and teaching males that expressing emotions is a good and masculine thing to do. Then when men feel angry, rage, disappointed with the world, humiliated, depressed, unloved and a failure they will be able to talk about it to others and then won't feel the need to lash out with a knife, bottle, fist or assault rifle.
The man who can express himself verbally...or in paintings, music, poetry etc. will be able to play Call of Duty in a healthy way and have lots of fun with it just as I played with toy guns as a child.