100 AG, Avatar Aang used Energybending to remove Ozai's Firebending correct? Ozai may have become jealous and extremely hateful of those who could bend. We also know that Ozai was not dead at the end of the series. He could have escaped and gathered anti-bender followers. Another idea is that Amon is the son of Azula. Azula's fate is unknown. Perhaps her bending was taken away too. If it was, she could have bore a son named Amon.
Roku said that some friendships are so powerful, they can last several lifetimes…does the same go for enemies?