How many countries have populations as diverse as ours? Most EU countries are fairly homogenous. Where they are not (Muslims immigrating to France, for example), there's problems. If anything, we probably have less violence due to ethnic and religious diversity than other countries.
Any equations should factor the drug trade. Take drugs out of the homicide numbers here in the US, and our homicide rates fall far behind other countries.
We should also look at the types of governments western nations have had since 1792. We've remained a constitutional republic. In that same time, various EU countries have been have alternated between monarchies (Britain, France), dictatorships (Germany, Italy, Spain, France), democracies (Britain, France, Germany, Italy, etc), socialist oligarchies (just about all of the EU), etc. I'd venture to say that the Second Amendment has had a fair amount to do with our government not devolving into something worse.