Though the entire Fire Nation in Avatar: The Last Airbender itself is presented as a massive threat towards the other three nations, certain members among them come off as much more threatening than others, and for very good reason. There's Azula, the sadistic and power-hungry princess of the Fire Nation, and Zuko, who's more of a misguided child led astray by Azula's influence as well as his desire to be recognized by his father.
Although the latter has completely understandable reasons for acting how he does before his eventual redemption, that doesn't mean Zuko didn't still do some heinous things, and Azula went on to continue her sadistic rampage far after Zuko began his redemption.
10 Azula: Destroyed Ty Lee's Home
Due to how blindly Ty Lee seems to adore and follow Azula after deciding to go along with her, it can be pretty easy to forget just how she was manipulated into doing so in the first place. While she was watching Ty Lee perform, Azula had the safety net set on fire, endangering the lives of the performers there- as well as releasing all of the troupe's animals, potentially endangering the audience as well. Because the circus was destroyed, it wasn't like Ty Lee had anywhere else to go.
It's safe to assume such a mishap didn't bear well for the circus' ratings, either, so it's highly likely that all of those performers lost their jobs- and home- because of Azula.
9 Zuko: Believing Azula Over Iroh
Since the very beginning of the series, viewers are shown that Uncle Iroh has wanted nothing more than to see Zuko attain a bright, happy future, meanwhile Azula consistently lied to, berated and manipulated her brother. Naturally, whenever the two are both trying to persuade Zuko to listen to them, he'd choose his uncle who truly loves him, right?
Of course, things couldn't be that easy, and one of Zuko's worst decisions was choosing to believe Azula, who'd never been truthful with him in the past, and betray his uncle who'd wanted nothing more than to subtly guide and watch over him.
8 Azula: Utter Apathy
Azula is shown throughout the series to possess a complete and utter lack of empathy for others, which is shown perfectly both in how a very young Azula spoke of Iroh handling his son's death, and how she reacted when she was shown up by Ty Lee.
She berates Iroh for not staying and 'burning Ba Sing Se to the ground' despite him clearly being consumed with grief after losing his only son. She also shoves Ty Lee to the ground after being shown up by her in gymnastics, despite her friend being willing to do absolutely anything for Azula. She makes it incredibly clear that she doesn't value the people she uses in any way.
7 Zuko: Returns A Kind Act With Thievery
During Zuko and Iroh's time in the Earth Kingdom, when they had to be on the run from the Fire Nation, they get taken in by a family who'd been incredibly negatively impacted by the Fire Nation and the effects of the war. Not only do they heal Iroh from being poisoned, but Zuko actually seems to develop a genuine friendship with the family's daughter.
This doesn't last, however, and that night Zuko steals the family's ostrich horses, and to make matters even worse, the daughter sees him do it- effectively shattering any bond the two had formed prior.
6 Azula: Attacking Iroh
At one point, Azula is effectively cornered by Team Avatar (who are joined by Zuko at this point) as well as Iroh, and being as cunning and remorseless as she is, Azula understood the only way to both survive and escape the encounter- but that doesn't make her actions any less heinous.
After falsely declaring her own surrender, Azula strikes a distracted Iroh with a precise blast of fire, which wounds him pretty badly before making her immediate escape. The lack of hesitation or remorse while severely injuring her own uncle is downright terrifying, and a great sign that family bonds don't mean anything to her.
5 Zuko: General Treatment Of Iroh Early On
If Zuko never got his redemption arc and learned how precious his Uncle really is to him along the way, he'd certainly be one of the most hated characters in the series. His treatment of Uncle Iroh early on is downright hard to watch at times, as Zuko unreasonably loses his temper with him, knocking over or destroying both his game pieces and beloved tea sets.
It's truly a testament to how pure-hearted Iroh is that he stuck around by his nephew's side despite how he was treated.
4 Azula: Disregarding Zuko's Life
Going as far back as to when Azula and Zuko were children, her treatment of him had always been unnecessarily cruel. For one, whenever Ozai states that Azula was born lucky meanwhile Zuko was "lucky to be born", Azula is seen smiling with pride aside her mortified brother.
She also toys with Zuko's feelings after his mother's disappearance, stating that "Dad's going to kill you!" in an incredibly cheery voice since the only one who'd clearly cared for Zuko's safety was now nowhere to be found.
3 Zuko: Siding With Murderers
During Zuko's desperate several-year journey to locate and capture the Avatar, he knowingly teams up with some pretty unsavory characters while attempting to do so. He comes across a band of pirates who are trying to hunt down the Avatar and his group for stealing a scroll from them, which they had originally stolen in the first place.
On top of being thieves, they also were later revealed to have accepted a large amount of coin to assassinate Zuko himself, which leads viewers to wonder just how many times they'd done something similar. Of course, Zuko was incredibly desperate, but this was still the wrong move.
2 Azula: Overthrowing The Earth Kingdom
While Zuko has to travel through the Earth Kingdom and keep his true identity a secret, fans are given quite the humbling look into the lives of the people of the Earth Kingdom and how negatively affected they'd been by the Fire Nation. Of course, this doesn't stop Azula (along with Mai and Ty Lee, who still followed her) from making things exponentially worse.
With the trio disguised as Kiyoshi Warriors, they were able to take control of the city's police force, and overthrow the Earth King with their help- igniting utter chaos in a kingdom that simply wanted to be left alone.
1 Zuko: Breaking Up With Mai Via A Note
Mai and Zuko's relationship was one that started from mutual childhood buds of affection for each other, blooming into a full-on romance later down the line, and he even helps bring out a warmer and more passionate side of Mai. That isn't to say Zuko handled everything about their relationship perfectly, and he particularly messed up when ending it.
Whenever Zuko decided that he was going to go against the Fire Nation and join Team Avatar, instead of trying to talk with Mai in person or express any of his true plans to her, he simply leaves her a note stating that he's breaking up with her- could the banished fiery prince get any colder?