Excuse me but what do you call this situation then?
MC trained Kendon for the simple fact of liking him and seeing his potential.. Kendon has surpassed him and now he feels so humiliated that he opens his legs to a demon all night to the point of affecting his tests and physical training, instead of to dedicate himself more to classes. All this to get your post back and be someone Kendon can admire again.
So.. what do you call it if not an inferiority complex?
I think it's more complicated than that. Which they didn't even resolve in the story since the raws are just 10 chapters. They gave such a heavy backdrop yet its mainly just smut. He knows that he can't ever be strong as kendon because he is half demon. This isnt fair for him. Still he tries a lot to be the person he'd respect and that's where the frustration stemmed from. And when they didn't talk after that night, it made him feel worse. And the others words that kendon will surpass him and ditch him must've resonated more now. Try understanding what he went through as well.
inferiority complex is the name of it (〜 ̄△ ̄)〜