Throughout Avatar: The Last Airbender it was apparent that the world of elemental bending had a rich history that stretched back even before the Hundred Year War that plagued the protagonists. While Aang's mission was to defeat the Fire Lord and restore balance to the world, there were numerous Avatars before him who faced other would-be conquerors and criminals, and many of them were every bit as cruel as Ozai.
Looking back over the original series, the comic continuations, and the spinoff books The Rise of Kyoshi and The Shadow of Kyoshi, there is a fascinating past filled with menaces and villains. Morality is complicated in a franchise as complicated as Avatar, but these villains represent the baddest of the bad before the Hundred Year War ever even began.
Tagaka the Pirate Queen
Though she preferred the titles "Marquess of the Eastern Sea," "Pirate Queen of the Southern Ocean," and "Waterborne Guardian of the South Pole," these were primarily attempts on the part of the Fifth Nation's leader, Tagaka, to paint herself as a hero rather than a monster. To her enemies, she became known as the "Bloody Flail of the Eastern Sea," and given her reputation, it's easy to see why. The line of succession ahead of her to inherit command of her pirate horde wiped out by Avatar Kuruk and his companions. Tagaka had scores to settle and the waterbending ability to do it.
Toward the start of Rise of Kyoshi, Tagaka serves as the first major antagonist, leading the Avatar into a trap under the guise of a peace treaty. In truth, the slaver hid her ships within icebergs she cracked apart with her waterbending as she attempted to take Kyoshi's friend Yun hostage. It was only through the manifestation of the Avatar State that Kyoshi foiled the villain's plan, and as far as fans know, she spent the rest of her days rotting in a Lake Laogai prison where she belonged.
The Chou Family
Villainy in the world of Avatar dates back to before the time of the Avatar itself, with the tyranny of the Chou family plaguing a prehistoric city atop the back of a lion turtle. Overseen by Chou the Elder and his three brutish sons, the nobles hoarded many of the resources of the city and protected it with weapons they barred from its people. When Wan crossed the Chous for the last time he was banished from the city, allowing him to negotiate with the lion turtle to keep his firebending permanently, thus starting his journey to become the first Avatar.
Though they weren't conquerors or criminals, it's hard not to see the villainy of the Chous as every bit as being bad as the series' other antagonists. Their violent demeanor came in the midst of a tumultuous time when their isolated portion of humanity had little idea what existed out in the world. Through jealousy guarding their position and maintaining a status quo, it took a legend to break the order of them and others like them. It was only then the world could know balance for the first time.
Xu Ping An
Even while Tagaka raided the waters of the southern hemisphere there were other threats in the world of Avatar. One of the greatest to plague the Earth Kingdom was Xu Ping An and his army of outlaws known as the Yellow Necks. Much like the Fifth Nation, the Yellow Necks ridiculed any followers of the law, bucking any such claims to authority that defied their own violence. With Xu Ping An commanding them they were a force to be reckoned with, and the Yellow Neck Uprising saw untold death that rattled the nation for decades to come. It was only with the intervention of Jianzhu they were stopped. He earned his nickname "Jianzhu the Gravedigger" because his methods to prevent them required such violence.
Even beyond his influence among the Yellow Necks, Xu Ping An was personally dangerous. Nobody found that out better than Avatar Kyoshi in a duel against the villain, in which he revealed his mythic ability to bend lightning, a skill even rarer then than in the time of Aang.
Xu Ping An very nearly killed Kyoshi with his lightning, furious after years of imprisonment in the Fire Nation with his abilities under clandestine study. Kyoshi's Avatar State saved her life yet again to stop him, but it took Xu Ping An's in exchange. He left her with scars she carried for the rest of her life, and the memory of killing Xu Ping An scarred Kyoshi perhaps even more deeply.
Chin the Conqueror
Xu Ping An would not be the villain Kyoshi was best known for stopping, however. That status went to Chin the Conqueror, the violent warlord who nearly commanded the entirety of the vast Earth Kingdom with his powerful legions and untold earthbending abilities. Viewing Chin's deeds in the grand scheme of history, it is possible to see his origins, as the Yellow Neck Uprising and similar conflicts with bandit hordes caused many in the Earth Kingdom to clamor for a strong and present ruler the Earth King simply did not embody.
But Chin's rule came to an end during his final showdown with Kyoshi. With his army at his back, Chin stood steadfast against the Avatar as he declared he would not lay claim to the peninsula beneath them. She entered the Avatar State and separated the peninsula from the mainland, forming Kyoshi Island and destabilizing the ground beneath Chin's feet. Stubborn to the last, the cliff Chin stood on crumbled beneath him and sent the warlord to his death, leaving only those in Chin village to lionize him as Chin the Great.
Toz the Cruel
The Earth Kingdom was not the only nation in the world whose history was rife with warlords and conquerors. Even in the time before Chin and Xu Ping An, the Fire Nation struggled to find any unity. The disaggregated region known as the Fire Islands had little protection against war chiefs and their armed militias laying claim to vulnerable villages, and the worst among them all was Toz the Cruel. Known for his unrelenting demand of tribute from those he conquered, Toz earned his nickname because even in times of famine he would threaten villages into paying him.
When one village protested, Toz ordered that all the children of the village be taken from their homes, and when they were never returned, the mothers of those children died in their anguish. Their deaths would not be altogether in vain, however, as the rise of the Kemurikage spirits haunted Toz and attacked his camps by stealing their children in return. Toz's forces abandoned him, and the people of the Fire Islands were so desperate to escape such terrible times they finally united under a Fire Lord for the very first time.
The pendulum of history would see many great and terrible leaders from that point on, but all would remember Toz as one of the cruelest of all.