Biology doesn't really have the concept of "fittest." Some individuals have more offspring than others. There is no necessity of selection by early death, although that does happen.
If a eugenics program were designed by science it would seek to maximize diversity rather than seek purity. Any animal breeder knows that inbreeding results in the more frequent expression of recessive genes, some of which are detrimental.
The Nazis had no interest in science. They were sloganizers.