As too why americans are so violent I think there are lots of reason's.
I think capitilism and democracy breed violence. These systems require people to feel that they are better than everyone around them, it's required. Thats the only way that you can condone stepping on peoples backs to "make it."
Communism and Socialism require you to feel that everyone around you is better than you. Not equal if they were equal then you wouldn't be required to give them any of your wages.
Also the US is a country founded on violence it's entire history is nothing but violence... as someone in the law enforcement field i would LOVE for there to be less guns on the street but the reality in America is that if they don't have guns they use knives, no knives they use clubs, no clubs they use ANYTHING. It's simply a violent socity on the most basic levels. Even people who say they would never hurt a fly regularly shoot the bird at people, cut people off yell and curse at people while driving... violence is EVERYWHERE here.
Don't think that europe is safe either as more counties become capitalist and more "free" the violence begins to rise. Compare statistics from 20 years ago to today and see if I'm not right!