You might want to take a look at Greg Mankiw’s way of addressing Jonathan Chait’s charge that Mankiw is a “social Darwinist”. I’d be very much interested in Mr. Chait’s definition of social Darwinism. I don’t plan to read the book to which he links and that seems like quite a bit of effort for so small a result.
My complaint about flinging charges of social Darwinism around are more practical. I think that taxing rich person A so technocrat B can redistribute the proceeds to rich person C on behalf of poor person D who may or may not actually benefit from the transaction is an odd sort of example of philanthropy, socialism, or whatever the opposite of social Darwinism might be. But that’s what our system does.
There are social Darwinists among conservatives and progressives alike. They just differ in who they think are “the fit”.