Nazi eugenics went beyond anti-Semitism to killing the weak and the mentally retarded, among others.
What Darwin did was to lay the “scientific” framework for the notion that man is just part of nature and therefore “nothing special.” If that is true, then killing men is fundamentally no more immoral than killing animals or, for that matter, destroying machines.
That is a moral argument that isn't contained in natural selection and not supported by Darwin himself. Natural selection is also responsible for cultural evolution that produces morals that say that killing is wrong and ultimately leads to the "Golden Rule", which predates Christianity.