Probably the breakup of the family seems the most realistic. I'm not big on blaming video games or music but when there aren't good parents around kids tend to grow up with very few values.
Gun violence is just another form of violence perpetrated by people who are not satisfied or angry about something in life and they choose to take it out on innocent people. In order for people to act so evil they have to set aside all real moral values. The values that used to be inserted by family and church have gone away with the downfall of these same institutions.