If a monster is a different race than another, it will attack the other monsters. The imp/demon, the werewolf, the goblin/orc, and the slime all seem to hate each other. I feel bad for the slimes.
I can’t help but think about possibilities of explaining why this is with a story, though it feels and looks like a bug to me.
I have a correction to make. The monsters will attack each other even if they are the same race. It depends on how close they are to each other. That’s a predator for you.
They seem to treat each other as another player character. They don’t kill the mice but will sometimes go push them around?