Dai Nana Maouji Jirubagiasu no Maou Keikoku Ki

The hero who was killed by the demon king ended up reincarnated as the son of the one who killed him! Thus began the life of the hero who donned in the mask of the ideal demon prince to topple his own kingdom while doing his best to prevent anyone from discovering his identity―― But to do that and blend into the demon tribe society, he had to kill his former comrades. Nevertheless, to become stronger, to save humanity, he had to walk through all kinds of paths of carnage! This is the tale of the demon race’s fake prince destroying his own kingdom.

TN: Most part of the other character in the novel is depicted from a non-human perspective so expect something that is completely different from the common sense of human.