About p 9 "king's advisor": Maybe "the king's right hand" would be better, given your comment?
The demon really misread the MC, thinking his "killer eyes" meant that he'd be a completely detached man, able to renounce the very thing he endeavoured himself so much to protect.
Also, is this demon really the strongest one is his country? Does "country" there mean the demon race as a whole or just a country within demon-kind?