Darwinism undermined traditional morality and the value of human life. Then, evolutionary progress became the new moral imperative. This aided the advance of eugenics, which was overtly founded on Darwinian principles. Some eugenicists began advocating euthanasia and infanticide for the disabled. On a parallel track, some prominent Darwinists argued that human racial competition and war is part of the Darwinian struggle for existence. Hitler imbibed these social Darwinist ideas, blended in virulent anti-Semitism, and--there you have it: Holocaust
Among German historians, there's really not much debate about whether or not Hitler was a social Darwinist. He clearly was drawing on Darwinian ideas.
Darwinism by itself did not produce the Holocaust, but without Darwinism neither Hitler nor his Nazi followers would have had the necessary scientific underpinnings to convince themselves and their collaborators that one of the world's greatest atrocities was really morally praiseworthy.
