Eugenics was the study of how to arrange reproduction within a human population to increase the occurrence of desirable heritable characteristics. Developed largely by Sir Francis Galton as a method of improving the human race, eugenics was increasingly discredited as unscientific and racially biased during the 20th century, especially after the adoption of its doctrines by the Nazis in order to justify their treatment of Jews, disabled people, and other minority groups. Using evolutionary biology language, professionals claimed that since evolution always leads to perfection, eugenics could help the human race by eliminating unwanted characteristics like genetic diseases and mutations, further expediting the evolutionary process towards said perfection.
Eugenics gave us a reason to disconnect society and science. The first movement was started under good intentions, to get rid of diseases and such. It was in the execution where the problem started. Forcing those who carry those diseases to not reproduce raises a lot of moral questions, as well as civil rights questions. Is their individual right to the pursuit of happiness and a free choice worth less than the overall health of the species? (The answer is no.) Also, perfection is subjective. We all know what Hitler thought perfection looked like. The power of something like Eugenics cannot be trusted in the hands of society.