From Giorgio Agamben, Remnants of Auschwitz: The Witness and the Archive
This is the specific ethical aporia of Auschwitz: it is the site in which it is not decent to remain decent, in which those who believed themselves to preserve their dignity and self-respect experience shame with respect to those who did not.
The answer Foucault give to this question in his 1976 College de France course is that racism is precisely what allows biopower to mark caesuras in the biological continuum of the human species, thus reintroducing a principle of war into the system of "making live." "In the biological continuum of the human species, the opposition and hierarchy of races, the qualification of certain races as good and others, by contrast, as inferior, are all ways to fragment the biological domain whose care power had undertaken; they are ways to distinguish different groups inside a population. In short, to stabilize a caesura of a biological type inside a domain that defines itself precisely as biological" (Foucault 1997: 227). [page 84]