Personally, I think blaming tv's, computer games etc is a kop out on the parents part.....
My son has had all these bad influences too, but hasn't felt the need to stab anybody, terrorise people or steal.... well, to my knowledge he hasn't
There were plenty of gangs around when I was growing up, most of them harmless. TV was restricted to 3 channels, and none of them started until late afternoon, but there was still cops & robbers, cowboys & indians, horror movies etc.....
Parents have to take responsibility. If a child is from a home without discipline or respect, it doesn't take a genius to work out that they'll grow up like that...