MC ends up in another world (transmigration, summon, game/novel/TV-become-reality, trolled by a R.O.B. or etc; is human or isn't as you choose, but was human before arrival). This world has a Human Empire waging a war of conquest on the continent/planet (space sector? if interstellar). They're the stereotypical aggressors, enslaving other races, "bad guys" as is more typical of Demon-lord faction in fantasy stories. Demons are actually allied with Elves, Dwarves, Beast-kin etc (or "demons" could be the world's version of Beastkin, going by the animal-type humanoids= demonkin trope), and the other types of races you choose to include. There may or may not be other humans outside of the Human Empire/among the other races, if so they're likely to be faced with hate-crimes, if not then the half-humans (half-elves, half-orcs, possibly halflings if their origin is half-dwarf or half-gnome or something of the sort) are the ones to face it.
MC has to deal with the moral quandary of the Humans being the "bad guys" of the world.