Scribe
Legend
In the real world? Even dogs know when they're being treated unfairly. Good and evil is grounded in biological instinct. But philosophers have debated that for millennia, there's not a good answer.
In the game? It's a vastly oversimplified construct used to categorize the bad guys. Why are they the bad guys? Because the game says they are. Which, of course is circular logic but it doesn't have to have any more depth than that in a game that uses constructs such as HP, AC or easily quantified abilities such as intelligence.
I don't play D&D to have discussions on advanced philosophical questions that will never be answered, I play it to have fun with friends, a few laughs and escape reality where I can't truly know who the bad guys are. Well, except for Bob. He's a bad guy.
Different people play for different reasons. I do think the MM should be far more explicit than it already is that the alignment in the MM is just a general suggestion for the entries that the adventurers are likely to face as opponents. But define good and evil? Other than "you know it when you see it", never going to happen in the real world.
This speaks to my point. I'm not even talking real world.
I'm talking about a world in which there are entities which may or may not be formed by the primordial forces of creation, which may or may not exist anyway.
I'm talking about a world in which Gods are real (or not) are champions for those primordial forces (or not) and hold meaningful power over the people of the world.
Or where Undead, can realistically be seen to be talking, thinking, moving, creatures.
What is 'Evil' in such a place?
Can you have a race created which inherently disregards what the Western World of Earth considers morally appropriate?
I'd almost think that would be a sure thing, assuming the other races of the fantasy world are not just humans in funny hats.