I do not know if it´s really that surreal and unrealistic.
I mean, the entire Inca´s religious beliefs were based on human sacrifices of the lower caste. They even have the Capacocha, where even noble families sacrifice his children because they were the best and pure.
The Karankawa and the Aztecs were known to practice cannibalism: warriors would eat a strip of flesh from enemies they had slain in combat. That was a symbol of status.
I'm just going to ignore these particular examples, since I just don't know enough about these cultures, but...
It´s not so strange the existence of entire civilizations that has "evil" practices that are tolerated and even encouraged by their society or social group in the real world.
It would be weird if there is not in a fantasy world.
You do not even have to search much about their motivations: The ethical and moral values of their society are what could be considered "Evil" by the standards of most other communities.
The Drow are much closer to what you describe as an evil society, however, I have an issue even with that, because I have a hard time seeing a society built on making everyone as miserable as possible survive long term without frequent revolutions, without everyone being inherently evil and predisposed to violence.
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As stated before, the Actecs, Incas, Celts, Karankawa, the Huns, the Mongols... even the Vikings or the Roman empire (Wich was built on conquest, pillage and slavery, but they were very good at assimilating cultures, public works and bureaucracy) beg to differ.
They simply focus their violence on other cultures or countries or slaves so those empires last for a long time.
You have a hard time believe it because you simply disregarded the examples of long-lasting societies that uphold behaviour that is considered "evil" in modern standards, as you said before.
And of course, there are people inside a society that do not comply to the standards, but the majority do not see it as awful because it´s not bad behaviour in his society to treat slaves or minorities as possessions, easily discarded, for example.
And if that happened in the real world in ancient times, why not in a (more or less) medieval fantasy setting?
Also, I agree with the notion of Orcs, White walkers, and other humanoid creatures being a metaphor about the beast vs the man, chaos vs civilization, etc more than creatures themselves so the ones you found are inherently evil because they depict an idea, not a race or a society.
Of course, there´s always the narrative that uses a particular individual that goes against the tenants of his evil society and represents the triumph of individualism and free will; that´s a good trope too. Plenty of characters use that.
