In an Unbelievable podcast discussing the question, "What was the primary cause of Nazi ideology—Darwinism or Christian anti-Semitism?" a listener asked, "Has Darwinism become prescriptive? When did this happen?"
Here is the argument I have heard many times against the Darwinism-created-eugenics claim: Darwinism is merely descriptive, not prescriptive. Therefore, those who made it prescriptive (i.e., who reasoned natural selection is true, therefore we can create better human beings through careful breeding, therefore we should) were misusing the scientific knowledge. This was not the fault of Darwinism, but merely a misunderstanding of the role of science.
The problem with these scientists of the past was that they didn't submit themselves to an ethic that values human life—an ethic that, in order to provide an "ought," would need to come from something outside of a naturalistic view of reality built only on non-prescriptive scientific observation and ideas about evolution. They failed to do so, with barbaric results.
Scientists today can clearly see what went wrong, and so they condemn those in the past. But if they now recognize that the mistake made by those past scientists was that of confusing the descriptive role of science with a prescriptive one, do they carefully guard themselves against this confusion today, or is their naturalistic scientism causing them to continue to make the same mistake? For if the scientific method determines what is considered to be real and therefore important, and science is only descriptive, then they can discover no moral prescriptives, and the descriptive becomes prescriptive by default.
Think of embryonic stem cell research. When scientists argue against people who oppose ESCR for ethical reasons, they don't usually counter with their own ethical arguments; rather, they charge their opponents with being anti-science. That is, they view the science of ESCR as prescriptive (what they ought to do), not merely descriptive (what can be done). Therefore, if you are against ESCR, they say you are against science itself.
But as scientists now admit was the case with eugenics, science has nothing to say about whether or not we ought to destroy human beings in their earliest state of development for the sake of scientific research. This is an ethic-of-intrinsic-human-value-vs.-ethic-of-instrumental-human-value question about how science ought to be used, not an ethics-vs.-science question. Just as they recognize was the case with eugenics, science is amoral. Yet they continue to make science their ethic, just as some did in the case of eugenics. Unsurprisingly, the results are less than ethical.
Scientists should learn from the past. It's better to stop giving science a prescriptive role and submit to non-naturalistic ethics now, rather than follow the amoral dictates of science and naturalism now and have to explain away atrocities later.