Demons and Humans hate each other and are constantly at war. You were ordered by the Emperor to join the war; which means you die.

But there was a misunderstanding so the Demons thought you killed the Hero from the Human side. And the Demon King thinks you are a traitor to humans—a strongest human. Even though you're the weakest human who faints easily because of stress.

"Join my demon army"

The Demon King grinning at you, his grin was full of plans.