One of the foundations of Hitler's social policies was the concept of racial hygiene. It was based on the ideas of Arthur de Gobineau, a French count; eugenics, a pseudo-science that advocated racial purity; and social Darwinism. Applied to human beings, "survival of the fittest" was interpreted as requiring racial purity and killing off "life unworthy of life." The first victims were children with physical and developmental disabilities; those killings occurred in a programme. After a public outcry, Hitler made a show of ending this program, but the killings in fact continued
Between 1939 and 1945, the SS, assisted by collaborationist governments and recruits from occupied countries, systematically killed somewhere between 11 and 14 million people, including about six million Jews,in concentration camps, ghettos and mass executions, or through less systematic methods elsewhere. In addition to those gassed to death, many died as a result of starvation and disease while working as slave labourers (sometimes benefiting private German companies). Along with Jews, non-Jewish Poles, Communists and political opponents, members of resistance groups, homosexuals, Roma, the physically handicapped and mentally retarded, Soviet prisoners of war (possibly as many as three million), Jehovah's Witnesses, Adventists, trade unionists, and psychiatric patients were killed. One of the biggest centres of mass-killing was the industrial extermination camp complex of Auschwitz-Birkenau. As far as is known, Hitler never visited the concentration camps and did not speak publicly about the killing in precise terms.