Being submerged in a certain type of social situation does not automatically mean one is part of it or partakes in the stereotype.
It was stated the youth was already troubled from the start due to reasons not disclosed, whether sociological, physiological, psychological or a combination. He apparently lost self-control and wanted to lash out at someone, anyone, whether guilty or innocent, just as any injured animal does when it has been hurt and feels threatened.
Playing certain games or listening to certain types of music doesn't change a person from a nice person into a killer or vice versa. That was there all along.
Listening and singing to religious music doesn't make you a wonderful, good, religious person. Listening and singing to country music doesn't make you a cowboy, cowgirl or country person. Listening and singing to opera/classical doesn't make you a learned intellectual or esteemed socialite. Listening and singing to metal doesn't make you a violent person. Listening and singing to rap doesn't make you a gangster. Life isn't like Final Fantasy X-2 where one can whip on a new outfit and automatically be whatever the outfit represents.
If listening to music caused this boy to kill or changed someone from a killer to a good person, then it would be a standard "treatment" and every prison and jail and rehab center and hospital in the nation would be piping certain types of music through the facilities 24/7 so people would be reformed, changed, healed. There would be ant-smoking music, anti-drinking music, anti-sex music, anti-overeating music, anti-cancer music. There would be music to heal injuries, illnesses. There would be music to make people more responsible, better parents, better kids. (No, I'm not talking about church hymns or christian rock here.)
There is the saying that music calms the savage breast. That is very true. The boy was likely drawn to Manson because it reflected his feelings, thoughts, and mood, not the other way around. Because even I at times do get angry and frustrated at someone or the world and turn on that metal and rock and sing and/or dance with it to get the bad energy out.