Well in complete honesty, I think it's the magnitude and the scope of it (hip hop - a partial factor).
Back in the "old days" I don't think it was so much influence from TV or movies - ie - Al Capone era (a bit far and a stretch, but an example nonetheless).
In days like that, it was more circumstance. You honestly think gangsters back then, watched a music video, filled with crap about guns, women and cars, then consciously went out and killed people for those reasons? (tv was just starting around back then, and the reasons weren't because of any media influence...they were self-fulfilling)
Same goes for Gotti, you really don't think he was doing it because of media influence do you?
What did these punks on Monday do it for? On a busiest shopping day of the year, out in the open.
Talk about a methodical plan (even then they're geniuses! for doing it at such a time
)
As for Arnold movies (and others of the same sort), back in the 80's, I don't think the gun violence was as bad (was it you who brought up "Ahnold" as the example?). It was fiction based, it wasn't standing in front of a mansion, preaching about life with 2 hot hoochie momma girls on either side of you, and huge bling-bling swinging on your neck. It was entertainment.
You also have to look at the ages of these kids now. Say worst case scenario, they're all 18, pushing 19. That leaves them the past 5 years in which they are starting to really soak up stuff they see, hear (and read, if they're doing that at all). They are making their own decisions (a lot of them stupid ones). They are growing up in a time where the media is pushing shit like hip-hop (the 50cent kind...personally I love Mase's stuff from back in 97/98, which he didn't go around touting guns).
I personally believe it got started just in the past few years and with the advent of the internet, where the ball really got rolling say, 6 years ago.
What these tweens/teens are being is exposed to a lot of crap, with some good stuff. They make the decision and the choice in what to watch and what they find appealing. In turn, believing what they want.