This is a re-post of one of my threads on 8chan /leftypol/ but since this board needs some fresh content I thought I'd post it here as well.

The answer is social Darwinism.

That's why the libertarians and Nazis on /pol/ generally get along just fine.

Libertarians would gladly have the "free market" economically purge undesirables, where as Nazis would just do it through the power of their military state.

Leftists believe in equality which is opposed to the idea of social Darwinism.

Ironically, most right wingers don't believe in evolution even though they advocate social Darwinist ideas whereas most left wingers believe in evolution although they don't believe that different classes of people should be discriminated against based on their place in the social hierarchy.

One could even theoretically argue that mutualists don't have a problem with Darwinism as long as it's collective Darwinism instead of individualistic "survival of the fittest".