it is revenge fantasy's series after all, of course they have to make everyone else as hateful as they could so the main can kill them without trouble.
It's not just that humans are a slave race, it's like every single member of every other race is the kind of person who likes to torture small animals for fun.
I find it hilarious that elves are so lazy to do anything that they left the work to slaves. I wouldn't be surprised if next time we hear about the Queendom that now there is a plague because of how awful the sanitation and vermin problem is.