The horrors of the 20th Century (Communism, Nazism) can reasonably be laid at the door of atheism and/or man's inherent sinfulness, but not Darwinism.
One of the insanities of Stalin's Russia was actually the repudiation of Darwinism. Commies believed in Lamarckian evolution, by which an animal's "struggles" would alter its offspring.
As far as equating Nazism's search for lebensraum at the door of Social Darwinism, to the extent there is a connection it is the other way around. Such genocidal struggles for territory and resources long predate history, let alone Darwin.
Finally, your post #208 makes a reasonably strong point that we shouldn't want Darwinism to be true. It says nothing at all about whether it is in fact so.
One of the insanities of Stalin's Russia was actually the repudiation of Darwinism.
As a famous Darwinist said "Darwin made atheism respectable". So yes, it can be laid at his door. Darwin was an atheist, his friends were atheists, the modern spokesmen of evolution are atheists. So yes, Darwinism is the scientific foundation for atheism. Marx wanted to dedicate Das Kapital to Darwin, but Darwin wishing to keep his atheism and his intentions hidden, refused. The founder of the idea of the master race, Haeckel, was highly lauded by Darwin and Darwin adopted many of his 'scientific' pronouncements (see the opening of the Descent of Man) as well as Haeckel's phony drawings. If the above were not enough the eugenics, the theory of inferior species, the idea of war for survival, the idea that it is okay to help natural selection by murdering people all came from Darwin. So yes, Darwin is the philosophical foundation for the horrors of Nazism and Communism.
I think it does indeed. A society without morals, a society where life is not valued, a society where man's life has no intrinsic value cannot thrive. We saw this pretty well in Soviet Russia.