And it is correct that Darwin didn't invent his theory of evolution as he modestly says he drew his ideas from Wallace, Galton, and Greg.
The ideas of the first two are probably better known than Greg's so I'll show part of one of Greg's essays in which he laments the failure of natural selection to cull the human race and what the ideal regime would be like:
“...Of course it will be urged that the principle of natural selection fails thus utterly because our civilisation is imperfect and misdirected; because our laws are insufficient; because our social arrangements are unwise; because our moral sense is languid or unenlightened. No doubt, if our legislators and rulers were quite sagacious and quite stern, and our people in all ranks quite wise and good, the beneficent tendencies of nature would continue to operate uncounteracted. No constitutions would be impaired by insufficient nutriment and none by unhealthy excess. No classes would be so undeveloped either in mind or muscle as to be unfitted for procreating sound and vigorous offspring. The sick, the tainted, and the maimed, would be too sensible and too unselfish to dream ‘of marrying and handing down to their children the curse of diseased or feeble frames;—or if they were not self-controlled, the state would exercise a salutary but unrelenting paternal despotism, and supply the deficiency by vigilant and timely prohibition. A republic is conceivable in which paupers should be forbidden to propagate; in which all candidates for the proud and solemn privilege of continuing an untainted and perfecting race should be subjected to a pass or a competitive examination, and those only should be suffered to transmit their names and families to future generations who had a pure, vigorous and well-developed constitution to transmit;—so that paternity should be the right and function exclusively of the élite of the nation, and humanity be thus enabled to march on securely and without drawback to its ultimate possibilities of progress. Every damaged or inferior temperament might be eliminated, and every special and superior one be selected and enthroned,—till the human race, both in its manhood and its womanhood, became one glorious congregation of saints, sages, and athletes:—till we were all Blondins, all Shakespeares, Pericles’, Socrates’, Columbuses and Fénelons. But no nation—in modern times at least—has ever yet approached this ideal; no such wisdom or virtue has ever been found except in isolated individual [362] instances; no government and no statesman has ever yet dared thus to supplement the inadequacy of personal patriotism by laws so sapiently despotic”
Of course neither Greg or Darwin lived long enough to see this eugenic Utopia come to fruition but they certainly would've had no difficulty understanding it and its roots when Mao or others sought to establish it.
Any nation or culture with a history of monarchy, oligarchy and colonization was well versed in "Natural Selection" as it applied to human populations. This dates back thousands of years and is not limited to Britain or Europe. To try to link Mao's cultural purges and the Holocaust to Charles Darwin is simply specious.