He learned how to survive in the darkness, how to make destruction logical, reasonable, and acceptable. The only thing he couldn't learn was how to take himself away from the darkness without hurting her. So he chose to go deeper. Not to become the king of darkness, but to be qualified to destroy darkness itself on the necessary day. And she was the only existence he was not allowed to participate in the destruction.
Intent: What "Advanced Black" ultimately wants to discuss is not the opposition between good and evil, but a more cruel question: When you have the ability to destroy darkness, can you still choose not to become the darkness itself?