Presumably we agree that: (a) Darwin didn't say that one race should go out and slaughter another; and (b) Hitler somehow came up with this on his own. Whether Hitler actually picked up erroneous bits and pieces of misinformation about Darwin's theory (or maybe something else) is irrelevant to the creationists' habit of saying that Darwin was the actual cause of Hitler's policies.
Read in context, it doesn't seem much different from standing on its own: Darwin puts it into the cool, rationalistic prose of science, but he doesn't seem bothered by the idea of one race trying to exterminate another. On the contrary, he observes it as just another fact observed in nature. Why then should Hitler hesitate to do what comes "naturally"?
Because, given the immense, deliberate, and frantic efforts Hitler put into his satanic enterprise, it clearly wasn't something that happened "naturally," as when a species goes exctinct over many generations because it fails to adapt to environmental changes. Shifting the blame for Hitler's genocide to Darwin's theory is clearly absurd.