Superior - Manga
Summary
The demon queen subdued enormous and powerful monsters and slaughtered over half of the world's population. Surviving members of the human race elected a hero to slay the demon queen, beseeching release from her oppressive rule. However, he questions his mission to eradicate the monsters: in his opinion, they too are conscious and alive, and so he anguishes over that fact. Sheila the Demon Queen became interested in him and joined his journey, hiding her identity and with the plan to kill him after she has gained his trust. Eventually, she realizes that she fell in love with the hero, and hence their journey begins...
(Part 1 ends with Volume 9)
Part 2 : http://mangafox.me/manga/superior_cross/
Living with the Loathsome feelings of Bitter Satisfaction is far better than helplessly watching the Corrupt and Self-Serving service their Greed at the expense of the very lives of others. He's all in on Revenge, not Justice. He isn't looking to give them what they deserve, he's looking to stop the monsters, permanently. Whatever, it's all Sophistry in the end.