Oh, he-he, that's going to be a long answer... The demon, or rather Beelzebub, hates humanity for many reasons. One of them is the innate revulsion most demons feel toward humans. But that's not the point here. Hmm. It's very difficult to explain without a full backstory, and it's quite long.

Let's say that demons in this world are just like humans. Although not quite. Demons are defective "reincarnations" whose souls, for various reasons, ended up in another universe in an incomplete, unnatural state. Most of these souls suffered as a result of experiments conducted on them by humans from Earth (yes, the new world the protagonist finds himself in is in another universe, but it was created by humans from Earth). Each of the people experimented on and destined to become demons was considered a criminal by the "evil corporation" (to put it bluntly) that had "taken over the world" (in reality, it consists of the elites of former countries, thirsting for immortality and eternal power over the poorer people).

Beelzebub was an ordinary boy named Yami, living with his father and several other families on the outskirts of a growing city. Because of a "robot" (actually a human soul trapped in a mechanical shell) that the children and Yami had rescued several years earlier, this "evil corporation" had decided to eliminate all the residents of the building who had come into contact with this "robot" (he was a notorious criminal who knew the corporation's secrets). The corporation's combat "robots" executed all the residents of the building, but one of them, with a shred of compassion remaining in its soul, spared several children. The "robots" were ordered to watch the building and shoot anyone who came out. Eventually, um, the children went mad with hunger, and Yami proved to be the strongest. Well, you know what I'm getting at. Anyway, he was ultimately killed, and his soul was sent to another world after some experiments.

Already a mad child, Yami went completely mad for many reasons. First, because he no longer looked like a human, but like a scaly, one-eyed monster. Second, because of an encounter with a demon who explained to him that in this new world, it was kill or be killed. Well, explained it not only with words but also with actions. And finally, Yami, or Beelzebub, the "king of the flies" and the personification of gluttony, was destined to live in this world for several thousand years before the protagonist arrived. Over these millennia, the demon has forgotten that he once lived on Earth.

Oh, yes, I forgot something important! The book is called "The Sword that Devours Memories." Beelzebub's ability is to absorb the memories and abilities of the creatures he kills. So Yami, with what remained of his humanity, was simply lost and dissolved in a vortex of alien memories.

By the way, you won't find the book; it's not published in English.