Our society has forced our children to watch many hours of television every day since they were born. The TV is a convenient baby-sitter. Through the TV parents can get the kids out of their hair. Parents submit the raising of their kids to the TV. They don't want the responsibility of raising children. They don't want their children to get in the way of them doing "their thing."
So, by the time a kid is in high school he has witnessed an unheard amount of violence.
He has witnessed so many thousands of murders he takes it for granted that people kill others every day. He has watched so many millions of people vent their anger on others he considers it a normal way of life. He has been bombarded with so many sitcoms that flaunt disrespect for parents and anyone else of authority he no longer responds to anyone of authority. He has lost the ability to separate fact from fiction.