As I see it, statements that "LOL nature decides" are rather inherently false if we speak of human civilization. Civilization from its earliest has tended to be rather against the interests of both wild animals and in fact gatherer-hunter societies who depend to a great extent on said wild game. We are no longer existing only with stone tools, throwing spears, and animal-skin clothing. What nature does/is is to a great extent irrelevant and a natural fallacy.

You are also right that Western societies adopted eugenics starting in the 19th Century. The foundations for that existed as far back as Hellenistic times when children deemed deformed or defective were exposed to die. Values Dissonance makes it hard for us to understand that that was not even considered a right to defend then, just part of the order of things.

You are also right that since the defeat of the Axis Powers by the Allies in World War II the democracies have done everything in their power to forget that eugenics was not just an aspect of fascism.