He could totally do something about it without making the country an enemy. Since Kawazu (1000 magic power) was the strongest and he has 8 million 1 thousand magic power, he could just completely seal their, as in all of the people involved, ability to summon people. It's neat, non-violent, and you can't really do anything about it except complain. Plus, it's not like they could say that he did something bad, he stopped them from sacrificing and enslaving people, unless the country loves that. Also if he isn't the demon king, he should meet the demon king. He's probably too strong for the demon king to do anything anyway so he could stop the demon king and get an new friend, win-win.