And to further tear your argument, Spanish who have a "reputation" in Europe of crossing by red light and of having a bit more of loose attitude towards rules have a similar murder rate. 0.9 for Spain, 0.86 for Germany.
I think the factors are external rather than internal. I don't know exactly what causes it though. The higher availability of guns in the US? The fact that the prison system in the US does not reform but punishes instead and convert first time convicts into hardened criminals? Other factors?
I don't know but having lived in all three countries, I don't see that many differences in term of people's character that could explain the difference in murder rate.