Ozai
In the episode "Zuko Alone" his origin is shown in a flashback from back when his father Azulon was Fire Lord. After Iroh's son dies from the war in Ba Sing Se, Ozai claims that he should inherit the throne instead of his older brother, stating that because Iroh no longer has a son, the bloodline of Fire Lords would end with him. Azulon, infuriated at the request, orders Ozai to kill his own son, Zuko, to understand the pain that Iroh felt. Ozai's wife, Ursa, knowing that Ozai would do anything for the throne, offered Ozai a plan to spare Zuko's life and his place to the throne.  This resulted the death of Fire Lord Azulon and the banishment of Ursa, though the latter is not revealed until season 3.