My headcanon is that Ozai was sexually abused by his father as a child and never got help, and this is part of the reason he ultimately had became such a monster.
I would say this is believeable both because Fire Nation is a pastiche of Edo Japan and Imperial Japan, first of which is one of the infamous examples of pederasty-tolerant societies, and the second of which is infamous for perhaps commiting more wartime rape over the course of a single war than any other society in history, and because from what little we had seen of him, Fire Lord Azulon seems like the type of person to whom it would be not out of character to molest his younger son.
Now live with this, you all.