This is Steven Pinker's response as to why America is more violent than other countries:
How does the United States compare to other democracies in the Western world?
The United States has a higher rate of violence, partly because large parts of the country were in a state of anarchy until the 20th century. People could not count on the government to protect them. The cliché of the cowboy movies is that the nearest sheriff is 200 miles away, so you have to protect yourself with your six-shooter. When governments did arrive, people were reluctant to hand over their self-protection. And because the first government in America was a democracy, people were able to impose their wishes. In European countries, first the government disarmed the people and exerted its control many hundred years ago. Then the people democratized the government, but by then the government had already established control.