England and France have far bloodier pasts than the US does though, Andy, and they don't have nearly the murder rate.
I think it more to do with the national mythology. The US is the wild west. Jesse James' distant descendant is on TV now and he's still an outlaw. Bonnie and Clyde were heroes to a lot of people, then when they were killed their death car became a tourist attraction. Even their law makers were basically violent vigilantes...think about the Earps or Pat what his name who murdered Billy the Kid. The US has, since it's very beginnings, cultivated a violent mythology.
Imagine if Canada celebrated the Mad Trapper and the Black Donnallys that way.