Dawin's theory of natural selection can't be invalidated because it it inspired eugenics, and perhaps it's illegitimate to bring up what the moral implications of Darwinism are in a discussion about its truth or scope. After all, it's entirely possible that Darwin's theory is true in every possible respect and, as Dennett, Dawkins and (according to Dennett) Eugenie Scott http://www.guardian.co.uk/commentisfree/story/0,,1746189,00.html believes, that both traditional religion and traditional morality must be considered false because of it.
At the same time, however, I don't think it's demonizing Darwin or Darwin's theory to admit that he thought it did have moral and religious implications.
Among the moral implications he thought the theory had were these:
I could show fight on natural selection having done and doing more for the progress of civilization than you seem inclined to admit. Remember what risk the nations of Europe ran, not so many centuries ago of being overwhelmed by the Turks, and how ridiculous such an idea now is! The more civilised so-called Caucasian races have beaten the Turkish hollow in the struggle for existence. Looking to the world at no very distant date, what an endless number of the lower races will have been eliminated by the higher civilized races throughout the world.
http://pages.britishlibrary.net/charles.darwin/texts/letters/letters1_08.html
Darwin had been inspired by Malthus, and Darwin knew and approved of Francis Galton's applications of his ideas.
If you ever read vdare, in some of the articles you'll come across similar theorizing based upon Darwinian natural selection as applied to race.
Now, I think it's obvious that Darwin was a brilliant guy, a good guy and no Hitler, period. After all, nobody blames Malthus for Hitler. But Darwin's theory has an obvious connection to eugenics and eugenics has an obvious connection to Nazism. To say that Darwin's theories have had no connection to eugenics would be like saying those of Mathus has no such connection.
The thing is, you're right that whenever Hitler is brought in, a lot of heat but little light tends to follow.
But the eugenics movement was one that enveloped nearly of America and Europe's intellectuals, not just the Nazis, and to their credit it was Christians like Booker T. Washington and William Jennings Bryant who stood against it while secular intellectuals like W.E.B. DuBois bought into it whole-heartedly.
That still doesn't imply that Darwin's theory of natural selection is false or even that its application to eugenics is misguided. But somewhat as Pope John Paul dealt with the relationship of Christians to Jews and clarified that the Catholic Church is not anti-Jewish, it seems to me that the relationship between Darwinist theory and eugenics is not a simple one to be brushed aside with an invocation of Godwin's Law.