It's a big part, my FRiend. Video games reward players for violent action. Rap music glorifies violent action. Depraved behavior is presented as normal, even admirable.
Most folks exposed to these things will not be adversely affected. But a small minority will. Think of the Germans in the 1930's who were exposed to relentless Nazi propaganda. Otherwise decent people got all twisted up the wrong way.
Now, I'm not arguing for blanket censorship, or anything like that. Perhaps in a free society we must accept certain things. But it's foolish to ignore what's part of the problem.
Most folks exposed to these things will not be adversely affected. But a small minority will.
OK - so now please provide evidence of the video games Holmes played & the music he listened to.
Or maybe he was made nuts via osmosis...the video games & music other people enjoy. Yeah, that's it!