I mean, I get it. The crown prince so far has done nothing wrong (except push for the marriage, which is pretty okay for political marriages, and cause those demons to come, but he is careful to not let anyone die), and is merely trying to survive, but even so, with pretty tame methods, comparesd to Letio. Letio, at the other hand, while he doesn't seem to be malicious, his moral code is more twisted than a spanish soap opera's plot. He sees no wrong with murder, and using people (that whole talk with the twin brother screamed "I'm gonna use you for her"), and as fl said, in his eyes, those who are good to him are good people, those who aren't, are bad. Even if they are "bad" because of his own fucked up actions. I mean, he starts a romance with that horned-hair girl, despite her being a taken woman, as if it's okay, and then suddenly shows up to steal the throne from the prince, who's survival hinges on it, so understandably he wants to take it bad, and suddenly that makes him the bad guy?
I think you are right, that the original novel suffers from a bad case of "unreliable narrator". Things, that letio sees as good are painted as good, but are they really? If we consider how he has no common sense, doubtful.
I got this feeling that letio is actually the villain or some sort? And he is just the main character of the story that the FL read because that's letio's point of view? You know, point of views are different with another person.
I can't explain it thoroughly but that's the point HAHAHAHAHA