i view it this way;
it's not that they've done nothing wrong, it's that they have an incredible amount of power that other people or leaders view them as a threat.
so the ruler of the ogasic kingdom recognized the demon king as someone who is powerful enough to pose a threat to him, so he makes the demon king into an antagonist
There are actual bad demon lords in stories like this, but sometimes they have normal reasoning, like they need resources the humans have for survival etc, they're not evil for the sake of it. I think because everyone has done the old formula already there are more stories now giving the demon's perspective and making them sympathetic (especially in romances)
I actually sometimes feel bad for the demon kings or others who are told to be bad. Have they done something wrong? Like if they have use *talk no jutsu* or maybe someone done something really really bad to them .No one wants to hurt other . They didn't want to be bad. Like for example ender dragon from mine craft have he or she done something wrong?they are just there. Have they every killed any of the pets? Have they burned your house?have they killed you? Or have they killed others? Like seriously whats the problem ? You are just killing them when they done nothing wrong . You just need to understand them .