Why must the devil be Defeated?

“The Goddess summoned me to this world to defeat the Devil that has imperiled the world and to restore peace to the world.” “I understand that it’s my cardinal duty. Since I’ve been summoned to become a Warrior, I’ll do my best to beat my opponent even if he’s the almighty Devil.” “Don’t ask me why. That’s what the book says.” “But… isn’t this development straying from the plot?!” This is the beginning of a scam in an alternate universe. It is also the story of a warrior with a mission to defeat the Devil. At the end, he realized that he had been duped instead.