User blog comment:DBard/Thoughts on True Longinus and God's Dying Will/@comment-1359322-20130725004228/@comment-1359322-20130912035549

[HAHAHA, that brat moved around without thinking about what he wanted to become, that’s why he failed. If he wanted to become strong as a human, then he shouldn’t have relied on Medusa’s eye. Since he spoke of being a hero while being half-assed that made him fail. As the result, that eye became fatal. Laughable, right? Laugh at him. At the end, he turned into a clown.]

Yeah, if he had continued to fight as a simple human, then the holy-spear would have lent its strength to his will.

So it ended for him because the “The will of the God from the Bible” determined that “Rather than the ambition of the host, the dream of the Sekiryuutei who is a Devil and a Dragon is better” huh.

(Quoted from Volume 14 Romania)

If we based on what the explanation given by Indra/Sakra and Azazel, it seems like it is because Cao Cao's downfall is because he went beyond the boundaries of a human while trying to maintain one.