Well that in itself is reason for the authorities to voice their concerns about the gaming industry. An industry that is under fire for the amount of violence present should really ensure that they certify their games accurately.
I am uncertain how one ascertains whether their child is mentally mature enough to constantly play violent games and not be affected by it on in some way? It would be interesting to find out whether many of these mass murderers parents ever suspected that they were capable of such acts.
Please note, I am not saying violent games are solely to blame but I suspect they may play a part. Hell, I'm an adult and play racing games, sometimes, just sometimes, when driving you think to yourself "ooh, I wonder if I can slide my car around the corner like I do in XYZ game"....