Laugh at it. He doesn't deserve your serious hatred. He deserves your mocking indifference.
If you are looking to "make a top" list or a guide for characters you love to hate in these types of angsty, "bittersweet" manga, here is how to structure it: Guide to Ranking Your "Hated" Exes nagi hikaru my exboyfriend who i hate make top
If you truly want him to be "top" in the sense of being your main enemy, you are giving him power. So let's reverse it. Laugh at it