Here is an interesting article about why these comparisons do not make sense -- the comparisons of US murder rates and so forth - basically saying how these stats and comparisons are biased.
Unfortunately for the article, the comparison of the US to other wealthy, developed countries does
make sense. We compare our education system to other developed countries, our health sytem to other developed countries, our life expectancy to other developed countries. We're not at the top of the list but we are not that far from the top in most categories.
Comparing our murder rates to impoverished nations with sky high murder rates doesn't make sense. Do we compare our life expectancy to countries where people see a doctor only a few times in their life?
So yes, our murder rate is moderate compared to most countries, because most countries are poverty stricken, as of this writing. But we compare every other aspect of our standard of living to the other fortunate developed countries, so to break ranks and start comparing ourselves to impoverished Third World nations would be showing bias.
As for diversity, it is true the diverse US will probably always have a somewhat higher rate of violence than homogenous nations where only one type of people have lived their since the Ice Ages. Not all of our newly arrived groups get along with each other. But countries like France and the UK also have a lot of immigrants from poor countries, they too are diverse, and their murder rates are about a quarter of the US rates. So it isn't just diversity that causes the problem.