No, they didn't kill "the retarded", but we had programs to sterilize the mentally ill, the retarded, and in many instances anyone who was not "of old pioneer stock"...
THis was how Margaret Sanger, and what became PLanned Parenthood, got it's start...
There's plenty of documentation out there on this (it's near and dear to my heart because of some people back in my family line who were forcibly sterilized because they were indians in Vermont, apparently a no-no), beginning with Indiana's Eugenics law in 1907, up until it finally stopped in the 60s completely...
``It is better for all the world, if instead of waiting to execute degenerate offspring for crime, or to let them starve for their imbecility, society can prevent those who are manifestly unfit from continuing their kind,'' Supreme Court Justice Oliver Wendell Holmes wrote in the majority opinion of a landmark eugenics case in 1926.
No, the germans may have been the worst practitioners, but they were by no means the only one. We can also point fingers at ourselves...
Forced sterilization of the retarded (USA and Europe) was not the same as killing them (Nazi Germany).