It’s quite the stretch to imply that all Hitler did was apply the general concept of “survival of the fittest” to humanity. It’s true that animals who are unable to find sufficient food or a mate before passing are unable to pass their genes to the next generation, but these changes are incredibly slight; even in the wild, most animals don’t die because of their genes - they die because they were unluckily eaten, or fell sick, or any number of other potential issues. Evolutionary changes through natural selection generally only happen over millions of years due to this reason. To say that most people suffer due to some genetic factor, and that their failure to reproduce is having a meaningful, positive short-term impact on the human species is not just overlooking virtually every sociological data point - it’s overlooking basic evolutionary theory as well; animals try to live in part because the more living different individuals, the better their species’ likelihood to survive.

In the wild it’s not uncommon for a gene that was heavily selected for under a certain environment to suddenly be ill-suited for a new environment. Mammoths were well suited to a cold ice-age climate, but died out relatively quickly when weather warmed up. It’s humans who attribute evolution to “upward” momentum, when really it’s simply lateral change over time, only becoming more suited to whatever arbitrary conditions the creature is currently subjected to, rather than improving any “objective” fitness.

In the end, genetic variety is the most effective way for a species to survive, because any number of different selective factors can suddenly make any particular trait essential for survival and reproduction. For example, baby seals with “ugly” coats ended up having an incredibly fortunate genetic variation when humans decided to massacre them for their fine furs, and now the species has survived in part due to something that otherwise would have been deemed by us to be a meaningless, or even undesirable, variation. Hitler’s actions were not natural selection - they were, like the baby seals, a form of artificial selection; the arbitrary culling of genetic variations that were seen, by one man’s opinion, as “inferior.”

If Hitler and other eugenicists truly believed that human suffering were simply the natural form of evolution, they would have no need to take action into their own hands; if people who are genetically inferior are destined to die, then simply let them - no need for concentration camps and genocide. Their talking points are nothing more than half-baked justifications using data that’s cherry-picked at best, and simply made-up at worst, to allow them to force evolution to take unnatural pathways. If they succeeded, it would simply bottleneck our genetic variation and make us less suited for any given environment - not more.