" My own guess is that Americans (particularly in the south and west) never really signed on to a social contract that gave government a monopoly on the legitimate use of violence, as Europe did. Americans not only retain the right to bear arms but believe it is their responsibility, not the government’s, to deter harm-doers. With private citizens, flush with self-serving biases, acting as judge, jury, and executioner, body counts can pile up as trigger-happy vigilantes mete out rough justice. This may be a legacy of the long periods of anarchy in the mountainous south and frontier west, and of the historical failure of the police and courts to serve African American communities."
[Citation Needed] - if the violence really was vigilantism, he's be able to site that, not just "guess."
However, he also points out that it's not "the guns," and removing firearms violence doesn't affect the numbers notably.