A recurrent theme that appears from time to time. Well, of course video games do not create criminality. However, to deny them any effect seems to me troublesome. Sociologically, if there are other conditions creating an absence of structure in one's life, video games, played hours on end every day, do influence one's behaviour. Maybe not to the extent of pushing someone to decide to murder, but in putting someone in such a frame of mind that anger, stress and lack of boundaries make the person unable to rationalise and control one's feelings and impulsions. And the problem with violent crimes is that they are acted upon impulse. So yes, I do believe video games can influence somebody, although they certainly are not the main factor triggering the person to act.
In that sense, the new study may well have flaws, like the others....
"New Research Shows No Link Between Violent Video Games And School Shootings" (TechDirt, 22 January 2009)
That said, the sociological influence does not fit within the legal definitions of what defences are. Thus, it is not a surprise if courts deny the claim any validity. Should they modify the law? I do not think so; this is criminal law and strong reasons of social policies motivate towards admitting defences only rarely. In this particular case, there is no abolition of consciousness.
'The Video Game Made Me Kill My Parents' Defense Rejected" (TechDirt, 13 January 2009)