The Cursed blessing

A world where numerous gates appeared and released countless monsters from their dungeons, but that didn't make humans extinct, Humans survived through that but a single man who was trying to find a reason to live was approached by a demon who possed a human's body, and asked to live for her. Which he did and when she asked him to kill all the humans he did so.

But that is not the context of this story, the human who got possessed was sent back 20 years before. This story focuses on that