Demihumans ARE monsters. You're trying to distinguish between "monsters that act like people" and "monsters that act like animals", but they're both still monsters.
Whether it's Slime, KonoSuba, Girls in a Dungeon, Overlord, or this, anything that's not a human is a monster. But that has no bearing on whether or not they're people.
The Wolves in Slime are people, they aren't just animals. Likewise, Kumoko is a monster, but she's also a person, she's not just some animal.