Because there are clear counterpoints to that. Take the Phillipines, or Mexico. Those countries have high gun ownership, violence, and are pretty homogenous too.

If you want a country with white people, take Russia (though I’m not sure how homogenous it is, it’s definitely not US levels of integration).

Simply because some homogenous societies have low murder rates doesn’t mean all do, and does not mean the reason is homogeneity.

Often white racists in the US make the argument that if there was more homogeneity then there wouldbe less violence. However this shirks the responsibility of the white people for the fact that historically they were the instigators of all racial hate. Black people in America have high crime rates due to systematic and concerted efforts to oppress them over 2 centuries. Blaming that oppression as just some natural unavoidable thing that happens when there is racial integration is what one is trying to do when they argue homogenity is the reason, rather than the belief of racial superiority or hate.

If you go to Countries like China, while they can say insensitive things about black people due to the lack of experience interacting with them, they definitely do not harbor any of the American style racial beliefs towards them.

Blaming lack of homogeneity as the cause for racial issues is shirking responsibility for causing them.