It isn't at all uncommon to refute Hume's Law. All naturalists are more or less bound to. Also, a change of facts will usually lead to a change of behaviour. If I yell "Fire" inside a building people will run out. Of course, I can claim innocene, since I never told anyone to run out, I only described the situation. But it would be a weak defence.
Their actions didn't come solely from your statement; their actions came from their own already formed general conclusions about things, such as "staying in a burning building drastically lowers your chance of survival" and "my survival is important to me". If you had someone who enjoyed the thought of burning alive, he would certainly not run away if someone yelled "fire" inside a building. Of course, pretty much everyone has conclusions like the first two, so everyone knows that most people will react in the same way if they yelled "fire".
Similarly, I'm not so sure you could say that stuff like the Holocaust happened because of the theory of evolution and natural selection and whatnot. When I learned about evolution in school, I certainly didn't come to the conclusion that all people who aren't white are genetically inferior and should be murdered. For a person to turn a descriptive statement into a normative statement, the person must add and implement their own ideas and conclusions about the world. There is nothing inherent in a descriptive statement that leads into a normative statement without the aid of anything else. I think it would be more accurate to say that it was simply a convenient catalyst that induced an already seriously messed up person into action.
Was it, perhaps, a contributing factor, such that without evolution, the Holocaust might not have occurred? Maybe. Was it to blame? I think that whoever it was was right on the money who said that Darwin was to blame for the Holocaust like Jesus was to blame for the Crusades.
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