As the Avatar, Aang is supposed to be the savior of humankind, capable of mastering the four elements and using them to help others. Katara points this out in the opening credits of each episode of Avatar: The Last Airbender when she says she believes that Aang can "save the world."
Despite his savior status, Aang has not always been a force for good. In fact, he has done some pretty heinous things over the course of the series. Some of his actions are relatively minor evils, while others have truly catastrophic consequences. These are ten times Aang acted like a villain.
10 Abandoning His Responsibilities
Generally, an Avatar is informed that they are the Avatar at around the age of sixteen, being allowed to live out their childhood in blissful ignorance of their responsibilities. However, Aang was told this truth at a much younger age. He panicked and did not handle the news well.
In fact, he ran away, resulting in him becoming lost in a storm, then frozen in ice for a century. This allowed the Fire Nation to attack and wipe out the entirety of the Air Nomads in an act of genocide without being opposed.
9 Animal Cruelty
Aang loves animals. He loves riding penguins as his personal sleds. He loves riding giant fish. He basically will ride any of the giant critters of the world. The problem is, he doesn't ask. So while he sees this as innocent fun, it does real harm to the creatures.
Having bonded with a flying bison at an early age, he may take for granted that animals not raised in captivity are not as willing to have humans jump on them. But the ways that Aang the all -powerful Avatar - treats these wild animals is nothing short of abusive.
8 Tormenting a Cabbage Merchant
One of the minor characters who repeatedly makes appearances is the cabbage merchant. He is never given a name, so fans have just taken to calling him "the cabbage merchant," making this his official title.
Time and again, he is seen traveling throughout the Earth Kingdom, trying to peddle his wares. A running gag through the series is Aang destroying the entire supply of goods this poor man has, tormenting the innocent cabbage merchant as he screams over his lost cabbages, slowly being driven to madness as the Avatar undermines his every attempt to run a business.
7 Hiding Important Military Intel
When Sokka, Katara, and Aang meet up with a member of the Southern Water Tribe, Bato, the first two rejoice at reconnecting with a member of their tribe, but Aang becomes resentful as he is left out of their social interactions. Bato was left behind by his fellow warriors after receiving grievous injuries from a Firebender and tells the group he has been awaiting a message from the rest of the warriors.
The messenger who comes actually gives Bato's message to Aang. In his jealous state, Aang hides this secret message. By delaying in his delivery of the message, he could have put lives at risk and undermined the war effort against the Fire Nation.
6 Abusing Others while in the Avatar State
Aang can enter into the Avatar State when his life is endangered or when he is pushed to his limits and feels helpless. In this state, he becomes more powerful, capable of using all four elements as he gains all the powers of his past lives.
On a couple occasions, Aang is pushed too far and enters into the Avatar State, at which point he rains devastation down on his opponents, including some who would count themselves his allies in other circumstances. Perhaps it is telling that the episode entitled "The Avatar State" climaxes with Aang losing control and wreaking havoc on General Fong's Earth Kingdom forces.
5 Hurting Katara
When Aang first tries to learn Firebending, he is desperate to find a master-no easy feat, considering that the Fire Nation is at war with the rest of the world and trying to hunt him down. He eventually finds his teacher in Jeong Jeong, an old man known as "the Deserter" for having left the Fire Nation's army to set out on his own.
Jeong Jeong explains to Aang that before mastering Firebending, he must learn emotional control through Waterbending and the stability that comes with Earthbending. Aang ignores him, brashly playing with fire and burns Katara so badly that her skin erupts in blisters.
4 Abandoning His Allies
During the Day of Black Sun, Aang and his allies attack the Fire Nation's capital, knowing that its defenses will be weakened by the eclipse, as Firebenders can't bend during a solar eclipse. Unfortunately, their attack fails and they are forced to retreat.
Unable to rescue everyone, Aang leaves many of his closest allies behind, knowing the horrors that await them as prisoners of war. This ruthlessness is an inevitability of warfare, but that does not change the fact that he knowingly subjects his friends and followers to endure torture, incarceration, and worse.
3 Robbing the Library
Wan Shi Tong's Library is one of the most sacred and important places in the entire world of Avatar. It is guarded by the spirit for whom it is named, the giant librarian owl Wan Shi Tong.
When Aang crosses the desert to reach the library, he learns that the spirit has become wary of humans, who apparently only travel to the library to gain knowledge that can be used to hurt other humans. So Aang lies to the giant owl spirit and does just this, even stealing from the library as Wan Shi Tong angrily brings the whole building down around them both.
2 Abandoning His Responsibilities AGAIN
Aang has struggled to master his abilities. The yogi Guru Pathik waited for Aang at one of the Air Temples to teach the young Avatar how to master his abilities, including how to master the Avatar State--something that is essential for Aang to defeat Fire Lord Ozai.
When Aang arrives at the Air Temple, he begins his training and quickly begins to master the basics, but at the most important part of the whole process, he abandons his responsibilities and quits his training. He does this to save Katara, but in doing so, he sacrifices his spiritual calling for his earthly attachments. He eventually does rescue Katara, but loses his connection to the Avatar State.
1 Endangering the World
As a monk, Aang is a pacifist and a vegetarian. Naturally, war does not come easily to him. After being told repeatedly that he will have to kill Fire Lord Ozai, Aang meditates and contacts his past lives. Each incarnation of his past self tells him he must kill Ozai--including Yanghcen, an Air Nomad who shared his monastic vows.
He ignores all of them. This almost results in his death and a genocide of the Earth Kingdom's people. He manages to pacify Ozai through Energybending rather than killing him, but that is as much luck as anything else. He had been cut off from the Avatar state and only regained it when he was struck in an old wound, reopening his chakras. Aang comes very close to being complicit in genocide.