Responses
Because they're racist against a species.
I mean just think about it; from their perspective they've only known slimes to be as intelligent, dangerous and mighty as a mosquito, and now they're dealing with one of them being the king of an advanced civilization.
Quite frankly it makes sense that you can't expect them all to instantly take rimuru seriously lol

The fact that they’re spewing all this crap out in public is sending me shouldn’t politics be all about being sly? Why the hell are you antagonizing a Demon Lord recognized by all other Demon Lords AND multiple other countries?