If you're looking for ethnic explanations, the evidence deosnt match the claim that Borderers drive the American homicide rate to any real degree. While the White American murder rate is higher than Britain's, it's less than half that of the overall American murder rate, which is largely driven by the homicide rate amongst African Americans, which is 7x that of whites.

Source: https://www.economist.com/graphic-detail/2021/05/21/how-amer...

Britain at 1.2, white Americans at 2.2, African Americans at 15.5