What I find shocking is that the judge believed the person who committed murder did not understand the consequences of his actions, but did not accept an insanity plea.
As I don't wish to be flamed I would also like to comment that I don't believe that video games cause insanity. Magical thinking is as likely to be influenced by books, advertising, cartoons, religion, comic-books (incidentally, I remember when comics were blamed for problem behaviours in children and adolescents before the focus of attention turned to videos, and to games), etc.
If someone does not understand that shooting someone could cause permanent injury (or permanent death) then they are a danger to themselves and to others, and should be treated for their mental illness. If the person in this case truly did not understand that shooting his parents would kill them then it is likely that he was suffering from a form of mental illness before he played Halo 3.