To be honest if that king/church never attack them a peaceful relationship between the king's country will have been done but nooo they just need to become stupid and kill some of rimaru's people and now this is happening tbh rimaru never even wanted to become a demon lord he was force to become one just because of the story that the witch say about milim.plus he said many times to milim that he dont want to become a demon lord because he know the problems that a demon lord will bring into the land to begin with.