Just like Charles Darwin did not invent Natural Selection and Evolution, his family members did not invent eugenics. As abhorrent as eugenics has been when practiced, many in the 19th century associated with it for altruistic reasons.
The Darwin family did a lot more than "associate" with the eugenics movement. They STARTED the Eugenics Society and the society was run by the Darwin family and their closest associates (people like Keynes and Huxley) until the mid-20th Century.