Another (more benevolent) reason why the Nazis started their eugenics project was their sincere desire to make Germans genuinely happy. The Nazis sincerely (and erroneously) believed that “physical drives psychological”. In other words, that the health of the body determines the health (and thus happiness) of mind, heart and soul.
Which meant that to make all Germans genuinely happy (and to make each of them make the maximum possible contribution to society), Nazis had to make all of them physically healthy (physically perfect, actually).
They were wrong. The reality is exactly the opposite – “psychological drives physical”. Western and especially Oriental monks, hermits and mystics proved beyond the reasonable doubt that the sheer power (and a proper state) of a human mind can radically improve one’s health. And even cure seemingly terminal diseases.
Besides, there were thousands and thousands of examples when the saints (monks, hermits, etc.) were perfectly happy while their bodies were not in a very good shape (to put it mildly). And thousands and thousands of instances when perfectly physically healthy individuals were deeply unhappy.
As for the contribution to the society, Nazis had right in front of them a vivid, loud and clear indication of the errors of their beliefs. Franklin Delano Roosevelt. One of the most successful (if not the most successful) presidents of the USA and the only one who has been elected to his office four (!) times in a row.
And a handicapped, partially disabled individual, whose legs were permanently paralyzed. He could not walk and had to move around in a wheelchair – and was elected President when he already could not move unaided.
FDR was a perfect example why eugenic beliefs and practices of the Nazis were not just plain scientifically wrong, but actually detrimental to prosperity and happiness of Germany.
Instead of using them for the benefit of its Fatherland, the Nazi system rejected (and sometimes even murdered) highly talented and productive individuals simply because they had certain physical and mental disabilities or belonged to the “wrong race”.
And thus deprived itself of significant (sometimes enormous) amounts of functional value. Which contributed significantly to their defeat in World War II and to the demise of the Third Reich.