It's Not a Job, It's a Calling.When Koro-sensei was a human, he was thought of as a hero, but became bored of his work. He was assigned by Aguri Yukimura to be a Demon King and live in a castle, since his predecessor (actually Neuro from Neuro: Supernatural Detective) retired from his position. Koro-sensei shortly became an octopus after wearing the Demon King's wardrobe. Koro-sensei's pupil, who came to vanish the Demon King, was assigned by Aguri to be The Reaper. However, Koro-sensei was still bored with his new position, with no one worthy enough to challenge him. That led him to become a teacher in order to train students to be warriors strong enough to defeat him.