(This question moved originally from RPGs and pertains to the D&D game where individuals and societies may have explicit allegiances to good or evil.)
A typical convention in fantasy settings are evil races; not demons, but mortal beings who casually act evil. They live their whole lives in ways we would consider reprehensible. They are not simply operating according to different moral standards we might consider objectionable: they idealize evil in the same way we idealize good. Such societies consider casually betraying superiors (and not, say, ritualized honorbound combat) to be an acceptable means of promotion. They treat everyone around them, even their own companions, as disposable playthings.
I cannot wrap my head around having an allegiance to evil. Evil is defined by being malevolent and causing harm to others. An evil creature is a sociopath. How do evil creatures form societies without being hypocritical? By hypocritical I mean they have stop being evil under certain regular circumstances: working together, gathering food, building shelter, making weapons, raising children, etc. In this case they're only evil some of the time and mostly towards outsiders... which is exactly how many human societies have functioned throughout history. They wouldn't be sociopaths.
My only real world contexts for "evil" societies is those which selectively applied their morality to justify slavery, genocide and so forth. Those societies didn't consider themselves evil or aspire to it: they didn't consider their victims to be human and thus worthy of humane treatment. No real societies that I am aware of acted anything like orcs or drow. No human society had murder-based social advancement, casual betrayal or unironically worshiped demons.
I can wrap my head around being so alien that your definition of good may involve removing free will, fatal rites of passage, causing some suffering now to prevent worse suffering later, trying to restructure or destroy the universe... but I cannot comprehend idealizing evil unless you are a literal demon who feeds on suffering and live in a plutocracy where said food doubles as currency. Not so much being funny looking but no different from humans in terms of basic needs.