The trouble is, for 99.9% of young males growing up, playing violent computer games is not going to turn them into gunmen performing mass murders.
Therefore, the blame cannot lie solely with the widespread availability of computer games, more the fact that due to a person's makeup/dna/personality/past experiences etc they could be influenced in any way by this.
I do accept this would be hard to explain to the family of victims of these gunmen but it is partly the parents' job to ensure their children are exposed to what they deem suitable for their particular child.