Responses

i think at the beginning he was just going to use the hero as basically a shield so that people wouldn't try to kill him for standing out like in his previous life.
eventually he actually started liking her and the other girls as precious friends. him being the demon lord is more like something he realized along the way at some point.
now he keeps protecting them because he cares about them, and even though he won't admit it, I do think he likes helping people that he has grown fond of. he is just being realistic about things and it comes out seeming rather pessimistic and apathetic.
HOLY SHIT
SO HE'S THE DEMON LORD SO THAT'S WHY HE'S PROTECTING AMIYU JUST SO HE CAN COVER BEING A DEMON LORD WTF