Hitler, like many others during his time period, believed that "survival of the fittest" meant survival of that which he admired the most, namely, the "Aryan race", as blond as Hitler, as slim as Goering, and as manly as Goebbels.
Blaming Darwin for Social Darwinism is like blaming Adam Smith for Robber Barons (which we have seen done by Creationists, as well).