I read a post this weekend that just blew my mind. Rick Stump just nails the idea of religion in AD&D and how/why the humanoid races are the enemies of Humanity.
Explaining a lot more about racial alignments in the Monster Manual, Part One: No, orcs are not ‘inherently evil’. Then why does the MM list them as ‘Lawful Evil” – they’re devil worshipers! Culturally, orcs belong to a number of devil-worshiping cults (thus, the clan names/banners) which is why they tend to hate each other in the absence of an outside threat. Same with goblins, etc. Gnolls? Demon worshipers. This also explains the much more limited powers of shamans.
I honestly have nothing to add to that post, as it covers it all.