It's not violent video games, it's not drugs, it's just guns..

What is the role of guns in Japan, the developed world's least firearm-filled nation and perhaps its strictest controller? In 2008, the U.S. had over 12 thousand firearm-related homicides. All of Japan experienced only 11, fewer than were killed at the Aurora shooting alone. And that was a big year: 2006 saw an astounding two, and when that number jumped to 22 in 2007, it became a national scandal. By comparison, also in 2008, 587 Americans were killed just by guns that had discharged accidentally.
Population of Japan: 127,817,277. Population of USA. 311,591,91. Gun-related homicides in the USA: around 12,000. If the USA were Japan: under 30.