Yesterday, I caught a radio show extolling one of the same old arguments against secular Darwinism. Said argument is essentially this: anyone purporting a theory of biological evolution removes God from human life. Without God, there is no foundation for moral behavior, and thus people will become amoral, ruthless killers. Further, he purports that accepting theories of biological evolution, especially human biological evolution, leads to eugenics and institutional racism, and all of the atrocious excesses that go with them. Then, of course, he proceeded to Godwin himself, asserting that atheism perforce leads to Nazism and the Holocaust.
Unfortunately, he makes several fallacies in the process, and his argument doesn't really hold water (as much as it holds another substance useful for the fertilization of crops). Let's go through it.
As we have covered before, humans are social animals, much like chimpanzees, crows, or wolves. We organize into bands, clans, and tribes. The social organizations we form all have rules: behaviors that are co-operative and pro-group survival are encouraged, those that are unco-operative or anti-group survival are discouraged. Our morals do not require an outside, abstract entity for their basis and enforcement; we are capable of getting along all by ourselves. There is no reason at all why a lack of a God-figure would reduce us to antisocial stealing, raping, and killing. And if the belief in a God is the only restraint on a person, something has gone very wrong for them.
Secondly, eugenics is not specifically tied to theories of biological evolution, Darwinian or otherwise. One of the fundamental breakthroughs popularized by The Origin of Species is natural selection. This is the observation that organisms that are not well suited by their environments tend to be culled, while organisms that are better suited tend to survive and reproduce. This concept of natural selection did not develop in a vacuum, it is a direct counterpart to the principle of artificial selection. Artificial selection is the principle that humans can select for certain traits in domesticated plants and animals. For instance, by allowing only the fastest racehorses to breed, over multiple generations racehorses tend to become faster. The fruits of artificial selection are all around us: domesticated sheep are docile, domesticated cattle give more milk, domesticated grasses produce many large seeds and have stiffer stalks (according to Diamond, one of the motivators for the adoption of agriculture in the Near East).
Artificial selection has been with us for 40,000 years, ever since humans learned to tame animals and plants and breed them for desirable traits. Eugenics is simply artificial selection extended to human beings. Autocratic regimes and classes believe that they own stocks of human beings, and can breed desirable traits into or out of the species that way. Darwin isn't needed: this is artificial selection at work.
The most fundamental fallacy at work here is argumentam ad consequentiam. Even if Darwinism or a theory of biological evolution were a sufficient condition for Nazism, they are not a necessary condition. At the same time, even if they led somehow inevitably to Nazism, doing so would not invalidate either as scientific theories. Scientific theories gain that status because they provide a description for processes occuring in the natural universe. That one does not like the possible consequences of a scientific theory is not proof against it; I dislike the fact that things break when I drop them, but that is not proof against any given theory of gravity or the field of materials science. Scientific theories do not require belief to be true (for some value of 'true'). That is the nature of science.
The insinuation that secularism is somehow responsible for atrocities is a pretty blatant post hoc ergo propter hoc. Sure, I'll grant you that many of the atrocities of the 20th century were at the hands of secularists; previous to the twentieth century, they were at the hands of those who profess to be religious. I'll see your Hitler and raise you a Richelieu, a Torquemada, a Charlemagne, and an Amaury. Any ideologiy can be used as a justification for atrocity, and no ideology is immune. Even today, I know of many devout theists that would burn the heretics and stone the sodomites if they could get away with it. It is not secularims that leads to atrocity, but ambition, tribalism, and human nature.