I think the problem all of us sane gamers have is that homicidal, insane people DO seem to play power games like D&D and DOOM/Quake/SoF. For the computer games, they probably use the cheat codes to feel truly GODlike, running around and killing things. And D&D is so open-ended that with the right (or, more likely, the WRONG) DM you can act out any power fantasy you want. Power addicts will naturally gravitate to arenas where they can percieve themselves as powerful.
What people (and the media) generally doesn't seem to understand is that the gaming medium is almost certainly not THE cause, or even A cause, of these psychotics disorder. They have already snapped and the only thing they want in life is to FEEL powerful, and when role-playing in an imaginary world stops being enough for them, they move on to the real world.
Saying, as some people do, that D&D incourages murderous, dangerous behavior is like saying that playing with squirt guns or playing laser tag will lead to murderous, dangerous behavior (though I've met people who believe that as well).