One interesting bit about the design decisions in OD&D and AD&D 1E was that for all of the monsters that exist in the game world - from dragons to mind flayers to drow - was that the most dangerous and powerful one in the generic game world was Humans. Seriously. Not the rank and file 1-4 HP commoners but the sheer power available to high level characters, especially high level spellcasters, that no other monster could reach. And with the inclusion of level limits on non-human races not even communities of elves - cool subterranean evil spider elves or not - would match the power and versatility of a group of suitably high level PCs.
This was, by accounts I've seen, a deliberate decision by Gygax, and one of the places where the games pulp fantasy origins (were non-human protagonists were pretty much non-existent ) shine out over the Tolkien influence of mixed fantasy race parties. He wanted humans to be the focus of the game (hence the level limits for non-humans making them unattractive), and he allowed that this made them both the best heroes and the best villains.
In putting together ideas for Castle Mordha I was trying to limit myself to the 1981 Basic D&D set. That presented just Lawful, Neutral and Chaotic as alignments, which when you dig into it presents a pretty harsh world - the 'civilized' races are beset on all sides by chaotic forces against which they are trying to both pretect themselves and carve out a large enough space to grow. Law - described as assuming life has a natural order and that obeying rules is the natural way of life - is a synonym for Good, even if a sentence or two later Moldvay gives himself an out by saying Lawful characters might not follow unjust laws. Lawful characters are supposed to act for the benefit of the group over the individual. Chaos, on the other hand, is assuming a lack of order to the universe and with it the assumption that laws are made to be broken, selfishness and individuality rule the day; Chaos is synonymous with Evil and chaotic characters cannot be trusted.
There's a lot of assumptions packed into that - way more than I want to do now - but I'm struck by how it's very much not the nine fold alignment system of AD&D as there is no Good/Evil access. Law is assumed to be Good, with some wiggle room for possible 'unjust' laws. Unfortunately when you look at actual history Laws, being societal constructs to not just enshrine base principles but to stabilize an existing social structure, are right at home in our greatest evils. Attempts to force the implementation of a 'natural law' in Soviet and Chinese Communism wrought incredible evils before (and most certainly during) the time that they were used as vehicles for despots to accumulate personal power. The Nazis seized power through a clearly legal process, then designed laws that things so evil that they're now a synonym for it. Institutional slavery - as vile, deumanizing and evil a practice as any other - rested on a firm bedrock of law as well as cultural tradition. These were not Chaotic societies: the Law was Evil. That's why I prefer the nine fold alignment system, even if I didn't adopt it for this game.
Of course, I still have to enter into the discussion of Good and Evil. The bad guys in the dungeon are Chaotic (operating in small groups where leadership is determined solely by personal power and the ability to crush or subordinate your enemies), and the PCs shouldn't have major moral qualms with killing them, but not everything in the dungeon is an enemy (given the random reaction tables this is another deliberate but oft overlooked design decision in D&D prior to age of location dungeons with built in narratives) even if they are Chaotic. Outside the dungeon there are plenty of Lawful people but they are hardly all on the PCs side.
Given the history of the Shankill Isles as a campaign location the obvious choice for Lawful Evil is Slavery. When I first ran games in this world I was heavily influenced by both the Slaver modules and Joel Rosenberg's "Guardians of the Flame" novels where the alliance of the Slavers and Wizards Guilds make up the main bad guys. So evil slavers are baked into the cake. The setting being essentially the Caribbean Island chain re purposed for fantasy works for this as well, not in the exported slaves from elsewhere but because they have a climate for plantation farming of coffee, chocolate, bananas and sugar, which are amenable to slave economies. These are 'outside the dungeon' issues, so I don't know if the PC will ever get into them, but at the time of the game they aren't 'illegal', so I will be interested to see how the players react, especially in the three fold alignment system.
All that blather aside, which human evils, which places where we humans are the monsters, have you fought in your games, and which would you like to tackle next?