Most leftists believe that some people are more equal than others, and when you don’t believe in traditional morality and you do believe in a vague utilitarianism, this gets translated into the idea that “inferior” people may be killed or a minority of people may be killed if that helps the majority. They are in denial about how much their movement is a spin-off from the old eugenics movement and how the eugenics movement was a central ideology of the white supremacists in America and the Nazis in Europe.
Hitler was a great admirerer of Darwin, and the survival of the fittest, which played right into his theory of the superiority of the Aryan race.