Somewhere in creation a valiant warrior saves a kingdom by defeating the Lord of Darkness when he leads an army against a sinful nation at the order of the gods. First the nation rewards him, and then the gods reward him by having him raise one of their children to adulthood.
What could possibly go wrong with having the most powerful adventurer of his generation raising the goddess child with the most potential in hers?
I mean it's not like she's going to grow up and take over Hell now is it?
…Oh wait. Oops.