Social Darwinism was used many times as a reason to persecute the weakest. Although this theory was not a cause in itself, it served many questionable actions European governments have undertaken in the 19th and 20th centuries, such as colonialism and imperialism.
It also served eugenics, a popular current in the early 1900s. It is described as a “biosocial movement which advocates the use of practices aimed at improving the genetic composition of a population". This so-called science promoted the idea of a superior race, and that inferior people should either be slaves or exterminated.
Eugenics led to many atrocities. From 1907, Some American states enforced laws according to which mentally ill people should not marry and that they should undergo sterilisation. Between 1907 and 1963, over 64,000 people were sterilised in the United States. And last but certainly not least, the Nazi government conducted eugenics programs in an attempt to maintain a pure German race. Between 1934 and 1937, an estimated 400,000 people were sterilised by the Nazi regime. The same regime killed thousands of institutionalised disabled through compulsory euthanasia programs. Here is a nazi poster promoting compulsory euthanasia.