The titular Avatar of Fire Lord Ozai, a tyrant who planned to burn the Earth Kingdom to the ground.
Not every Avatar has been as peace-loving as Aang, whose pacifism is more tied to his Air Nomad culture than to his status as the Avatar. There have been fearsome former Avatars like Kyoshi, who killed the warmongering Chin the Conquerer when he tried to invade her home. And in the sequel comics, when Aang fears that Zuko's rule as Fire Lord is turning out the same way his father's did, the spirit of Avatar Roku pressures Aang to kill Zuko in order to stop him.
Aang definitely struggled with moral decisions in Avatar: The Last Airbender, and there were times when he accidentally hurt people - even his friends. As the Avatar, however, it's ultimately his responsibility to maintain peace and balance between the Four Nations. Here's why Aang, and every Avatar before him, has used their powers for good.
The Avatar Has A Spirit Of Light Attached To Their Soul
Avatar: The Last Airbender co-creators Bryan Konietzko and Michael Dante DiMartino avoided providing an explanation for where the Avatar came from throughout the original series. However, sequel series story of the first Avatar, as well as the true nature of Aang's soul. The Avatar was created almost 10,000 years before the events of The Last Airbender, when a human called Wan fused his soul to a spirit called Raava. Before bonding with Wan, Raava's role was to maintain balance between light and dark in the world, and to keep chaos at bay. Raava is first and foremost a spirit of light and goodness, and Wan himself (though flawed) also freely chose to fight the imbalance he saw in the world and stand up for the downtrodden.
Because every new Avatar is a reincarnation of the previous Avatar, Raava's inherent goodness and Wan's determination to help people have been ed down through the generations - even though different Avatars have taken different approaches to the goal of peace and balance. But while there's never been an Avatar that turned out evil, there has been a Dark Avatar.
The Legend of Korra's Dark Avatar
Before bonding with Wan, Raava's role was to battle a spirit of darkness and chaos called Vaatu, who was effectively Raava's opposite in every respect. Wan and Raava managed to trap Vaatu inside a magical tree in the spirit world, but he was set free during The Legend of Korra season 2 by Korra's uncle, Unalaq. Vaatu bonded with Unalaq in the same way that Raava had once bonded with Wan, creating a Dark Avatar. Whereas the Avatar's role is to be a light for the world and to maintain peace and balance, the Dark Avatar's role was to spread darkness and chaos. Fortunately the Dark Avatar was short-lived, as Korra was able to destroy the creature that her uncle had become.
