The creator's hobbies is creating mortal species, worlds, dimensions, and observed them. After some time he find himself creating too much to look after, he start creating divine being to help him lessen the work load. Aesir is one of them.

Demon king: Oh god, Why did you create me and then create the hero to slay me?

Aesir: I didn't create anything! I'm a GOD, not a prophet! I can't see the future, how am I suppose to know that you will turn into a demon king and the hero will be born !!! 

Author note: still testing the concept of the story. Will continue or change the direction depending on feed back.