I think for the first time, I'm perturbed by the setting. Before, I was pretty chill with even some of the more outlandish stuff, enough details were consistent about the series background that it more than satisfied me. And the author and/or mangaka are also pretty detailed about certain things, like geography.
But this time, I have to question why the Demon race isn't already at war. If a Demon Lord who rules just one of the territories/countries in the demon realm can crush a human empire like this, and would be triggered to do so if they found out their people were being enslaved or executed....that doesn't make sense to me.
This is not just about why hasn't any war started yet. In order for the humans in some countries to discriminate against demons as a matter of policy or culture, there needs to be a history between human and demon realms. In order for there to be a history, there needs to be a longstanding status quo and interaction. For that status quo to exist, there needs to be an understanding of the capability of the demon realm, even if the demons were isolationist. Something doesn't add up regarding the history of human-demon interaction.
It's probably explained in the LN, but at this point of the LN I've not read further.