Avatar: The Last Airbender has seen quite a resurgence of popularity within the last year. Since its addition to the Netflix library in May of 2020, Netflix has started production for a live-action adaptation of the popular series. With the recently widely praised casting choice announcement of Paul Sun-Hyung Lee as Iroh, the franchise indeed has a bright future. Ignoring the monstrous flop that was M. Night Shyamalan's movie, fans both old and new are looking forward to a more faithful adaptation of the beloved franchise.
The rich world-building and character development in the original series have prompted some imaginative speculation from the growing fanbase. One unnerving speculation suggests that the Fire Nation ruling family, much like all ruling families, may be hiding a disturbing secret. Reddit user u/all_thetime suggests that Azula and Ozai's erratic dispositions may be a product of generations of inbreeding.
With the exception of Azula’s mother and Ozai's wife Ursa, every known member of the Fire Nation ruling family was a skilled firebender. Even as a non-bender, Ursa was a direct descendant of Avatar Roku. She was sought out by Azulon in hopes of creating powerful heirs. If anything, this shows that the Fire Nation is VERY particular with whom they share their bloodlines.
Speaking of ruling families, the Targaryens have quite a similar story. It is public knowledge that the Targaryens "preserved their own bloodlines" and had the tendency to go mad with power. As stated in Game of Thrones, “Every time a new Targaryen is born, the gods toss the coin in the air and the world holds its breath to see how it will land.” Daenerys' destiny of becoming the Mad Queen, much like her notoriously mad father, is a huge plot point in the show. Madness and power-hungry royalty seem to go hand in hand, and intermarriage just may be the catalyst. There are also plenty of real-world examples of incestuous intermarriage throughout history. Still, within the Fire Nation, it is clear that conquering through fear and absolute power will always take first priority, even if it meant sacrificing family. Ozai and Azula can be seen doing this at the expense of both Iroh and Zuko.
Yet, not all members of the Fire Nation ruling family ended up ruthless. Iroh and Zuko were well-loved by all at the end of the series. Even so, Iroh left behind a dark past as the Dragon of the West. Having had a notoriously hot temper and with his attempt of capturing Ba Sing Se, even Iroh wasn't completely without faults. Zuko also left behind a past of mercilessly trying to capture the Avatar in order to take power and please his ruthless father.
Although there is evidence that supports this theory, there is also evidence that suggests that bending is actually spiritual instead of genetic. Katara and Toph are the only known members of their families that can bend. Toph's blindness was also said to have aided in her extraordinary earthbending skills, which King Bumi mentions is due to having a more neutral jing. The presence of twins Poi and Ping in the Earth Kingdom also supports this countertheory. Poi and Ping are identical twins, but Poi can earthbend and Ping cannot, demonstrating that bending cannot be genetic. Still, Azulon arranging a marriage with the descendant of Avatar Roku for powerful heirs proves that, at the very least, the Fire Nation thinks that bending can be influenced genetically.
The PG rating of the show makes it highly unlikely that any confirmation of this theory will ever happen. Still, one would only need to look historically to see ruling families often resort to genetically influencing their heirs to ensure power. Although disturbing, this theory isn't exactly a completely outlandish assumption. It may just prove that too much power can drive a person mad.