Culture, my friends. It all comes down to the morals (or lack thereof) that get passed down from parent to child within a given culture.
Cultures that teach The Golden Rule & uphold the rule of law tend to be more successful & less violent than those that lionize rapsterism (the beat that moves gangsterism) & its concomitant criminality. Compare, for example, Israel, which has the largest number of guns per capita of any nation, but 1 of the lowest rates of (peace-time) gun-related crime; to the US, which has far fewer guns per capita, but far more gun-related violence. And, to take it a step further, democracies tend not attack each other, while autocracies & theocracies often do.
I had an enlightening conversation today with a gentleman who emigrated to California from Mexico in 1970. He twice referenced 'his country' [Mexico], which immediately struck me as odd for someone who has been living in the US for over 30 years! When I asked him about this, he said he is 'still Mexican'. This stands in stark contrast to the millions of Asian & European immigrants who come to the US primarily to 'be Americans'. And much the same is occurring with many so-called asylum seekers entering the UK & other parts of the EU.
Something has definitely changed & it does not bode well for long-term homogeneity.