The scary thing about the eugenic movements of the late 19th and early 20th Centuries is not how easily they slipped from the laudable goals of promoting healthy families and avoiding reinforced genetic problems into the nightmarish realms of racial purity and forced sterilizations, but freaking
widespread they were. This was not just a handful of kooks and racist madmen, but pretty much everyone who was a mover-and-shaker (and thinker) of the era. Indeed, the number of politicians, philosophers, humanitarians, "thought leaders," and men of science who
weren't in favor of eugenics in some form or another is shorter than the list of those who were.
That those folk were, at the same time, considering their own particular ethnicity and background and belief system and "class" as normative and what eugenics should be promoting is, of course, mere coincidence. Eugenics was easily adapted into racism, nativism, classism, and tribalism.
And, not surprisingly, even though the name and most of the concepts from eugenics are shunned today, there are powerful echoes of the movement that still can be heard in the racism, nativism, classism, and nativism of the current era.