Was Darwin a Nazi? Of course not. But did his ideas form a causal nexus via Galton to ideas that would be integrated into Nazi policy? Yes.
Darwin’s racist and pro-eugenics thinking, combined with some implications of his theory that he expressed, laid the groundwork for Hitler’s diabolical outlook.
Ideas do indeed have consequences, but not all ideas play out the same way or weave their way in the history of ideas toward the same destination.
O’Connell and Ruse’s failure to engage deeply and fully with the historiography of this question makes it hard to take their effort seriously.
The authors imply that social Darwinism was a position taken by non-scientists who just didn’t understand the science.