The Awakening" Avatar Roku appears before Aang and tells him that Aang had inherited his mistakes here we finally learn what were the mistakes Roku had made.
10
"Perfect"
The story of Avatar Roku and Fire Lord Sozin gives Aang and Zuko insight into their predecessors' mingled pasts. As the episode begins, both living characters receive messages---Aang is visited in a dream by Roku, who tells Aang to go to Roku's home while it is the summer solstice, where he meditates to meet Roku in the Spirit World. Zuko, meanwhile, finds a note outside his door, telling him that he must find out about his great-grandfather's death, in order to understand his own destiny. Zuko then discovers a secret message in the note, which says that the secret is kept by the Fire Sages, whom Zuko tracks down, finding the secret "Last Will and Testament" written by Sozin.
In their youth, Roku and Prince Sozin were best friends, and they even shared the same birthday, which they celebrated together. On their sixteenth birthday, Roku was informed of his destiny by the Fire Sages, and the two were forced to separate as Roku went to learn mastery of the three other elements. Before leaving, Sozin gave Roku a gift of the headpiece worn by the Crown Prince. Afterwards, Roku went on his journey and trains with members of all the Four Nations. At the Southern Air Temple, Roku learned to master Airbending, and made a close friendship with Monk Gyatso, much to Aang's surprise and delight. Roku tells Aang that some relationships transcend lifetimes, as Gyatso proved when he continued his friendship with Aang.
After mastering Airbending, Roku traveled to the Northern Water Tribe and the Earth Kingdom to master the other two elements and fully become the Avatar. Twelve years later, Roku returned to the Fire Nation, where Sozin had become the Fire Lord. Roku wound up marrying his childhood sweetheart, Ta Min, with Sozin as his best man. At the wedding, Sozin commented that the Fire Nation was going through a period of great prosperity, and told Roku that together, the two of them could spread it to the world by expanding the empire. Roku was greatly disturbed by this, telling Sozin that the Four Nations are meant to be kept separate and that he should not think of this again.
Years later however, Roku discovered that Sozin had set up colonies in the Earth Kingdom, and went to rebuke him. Sozin, angered, attacked Roku, and the two engaged in a battle, with Roku finally destroying most of the palace and defeating Sozin, sparing his life in recognition of their past friendship, but warning him never to move forward with his plans again, lest he be killed as punishment.
Twenty-five years later, Roku's Island is consumed in a volcanic eruption so massive that the Fire Lord can see and feel it from a hundred miles away, and flies in on his blue dragon to assist his old friend. The two use their bending to try to contain the lava flow. While running away from danger Roku is overwhelmed by poisonous gases. Even though Roku calls to Sozin to help, Sozin leaves him laying on the ground, explaining that now that Roku is out of the way his plans seem possible again. He flies away on his dragon, leaving Roku to die. Roku's red dragon, Fang, encircles him to protect him as hot ash rolls buries and kills them. Shortly afterwards, Aang is born to the Air Nomads. Sozin's testament then ends, explaining about his later attack on the Air Nomads, but as Aang manages to escape, he spends the rest of his life searching, so that he can find and destroy "the Last Airbender".
Awakening from his vision with Roku, Aang tells his friends that the message of the story was that, as Roku and Sozin were both Fire Nationals, any person is capable of great good or evil. The group takes comfort at Roku's words that friendships can transcend lifetimes. Meanwhile, having finished his story, Zuko angrily goes to the Fire Nation jail tower and bursts into Iroh's cell. He accuses Iroh of having sent him the message, and then angrily demands the point, as the testament did not even reveal anything about Sozin's death. Iroh explains that the message did not refer to Zuko's paternal ancestor, Sozin, but to his mother's grandfather, and reveals this to be Avatar Roku. And that their combined heritage is the reason why Zuko was still conflicted over his moral dilemma. He then produces the lost crown prince's headpiece and gives it to a stunned Zuko, explaining that because of his heritage, Zuko has the power to redeem their family and the Fire Nation by restoring the balance in the world.