Eya didn't destroy the world herself because she can only create, not destroy.
Since Eya created the world, her being a god of creation of sorts would make sense. If she can only create, she would have to rely on someone else to destroy it, so she created the sword and the concept of the Hero to do it for her.- Also, maybe the reason why Eya doesn't just create some sort of entity like Eyala to do the job is because she holds onto hope that the universe can be saved, so she lets someone mortal do it so they can be stopped if a way to keep the universe alive comes up.
