Never expected this movie to be accurate...
They did not only make assumptions about Hitler, but early on they (The French, British and also Americans) did not disagree that much with the German eugenics practice. There were even articles published in respected medical journals in which American doctors decried that they were lagging behind Germany with respect to forced sterilisation. See for example: Eugenic Sterilization and a Qualified Nazi Analogy: The United States and Germany, 1930-1945 (sadly behind a paywall)
In the eugenics program they first sterilized and later developed methods to mass murder "disabled" people. (with gas, first with carbon mono-oxide from exhausts later with the pesticide Zyclon-B) They later used the methods of the eugenics program in the concentration camps as part of the final solution.
Research is to see what everybody else has seen, and think what nobody else has thought.