Yoshihiro Togashi's iconic work, Hunter X Hunter, has a plethora of themes, one of them being the exploration of people's humanity, more specifically the thin line they can walk between having humanity and swiftly losing it. While all of the characters experience this journey in one way or another, few others represent the dichotomy as well as Killua Zoldyck.
Born into a generational family of well-respected assassins, Killua and the Zoldyck name he carries are feared throughout the land. It isn't until he begins his journey in the anime and meets his best friend, Gon, that Killua understands the positives to retaining his humanity, beginning to find himself in a battle with his conscience.
10 Showed It: When He Met Gon
Due to his solitary lifestyle and terrifyingly overprotective mother, Killua had always failed to make meaningful friends, but in the face of all of his failures before he still decided to take a chance on a friendship with Gon.
This tiny moment of humanity for Killua, that appeared before his journey had even truly begun, was ironically the first step to him showing his humanity a multitude of other times throughout the series. Arguably if Killua had never allowed himself to befriend Gon, he may have remained an unfeeling assassin forever.
9 Almost Lost It: Killing Bodoro
In the final stage of the hunter exam, Killua faces off against Gittarackur, who turns out to be his older brother Illumi. Illumi exerts a powerful aura over Killua that forces him to back down from the fight, but it also leaves the boy haunted.
When Leorio and Bodoro step up for their match afterward, the pair isn't given a chance to face off, as Killua impales Bodoro from behind and forfeits his place in the exam. This moment of bloodthirst came from the intense feeling Illumi instilled in Killua, leaving him so terrified he abandoned all humanity.
8 Showed It: By Bringing Palm Back To Her Senses
In the NGL, when facing off against the chimera ants, one of Killua's allies, Palm Siberia, gets caught by the ants and subsequently turned into a hybrid creature. Killua is the first to come face to face with her new form when they infiltrate the palace, but his attitude towards her saves both of their lives.
Palm has become murderous due to the influence from the royal guard Shaiapouf, but no matter what attacks she throws his way, Killua chooses to defend. Eventually, he breaks down in front of her and cries, mentioning Gon and how awful it would be for him to see her like this. This display of humanity and suffering brings Palm out of her trance and assures both of them can live.
7 Almost Lost It: After Losing Against Chairman Netero
Killua and Gon's first encounter with Chairman Netero cements him as a key figure in both of their lives for very different reasons. Gon sees the playful game they play together as a challenge, staying up all night to beat the legendary hunter, but Killua sees his failure as a pain, a disappointment.
When he exits the room where they played, he remorselessly slaughters two unsuspecting exam contestants to try and counteract the frustration he feels. It's a shocking moment for Killua, even with his assassin background.
6 Showed It: His Protection Of Alluka
Alluka is Killua's younger sister whose incredibly intense and mildly complicated ability means almost everyone else in the Zoldyck family has condemned her to a life of solitude. Killua, however, is not scared of his sister, he loves her dearly and battles to keep her safe when he takes her to heal Gon.
Multiple times Killua is told that the family would never harm him, but Killua rebukes them all, saying it means nothing to him if they're willing to harm Alluka instead. Protecting Alluka continuously puts Killua in more and more danger, but it all means nothing as long as he can keep his sister safe.
5 Almost Lost It: Knocking Zushi Out Of The Ring
Killua meets the tiny but powerful Zushi with Gon when the two enter Heavens Arena, all three boys looking to test their strength against a variety of different opponents whilst gaining some money. Killua is more than aware that Zushi is weaker than him, but when the other boy proves himself to not be as weak as he first thought, Killua isn't happy.
He punches Zushi out of the ring without thinking, terrified afterward that he had done serious damage in a moment of rage. Luckily for him, Zushi was protecting himself with an unknown technique, but it could've ended much worse.
4 Showed It: Saving Gon From Neferpitou... Twice
Neferpitou is arguably the biggest antagonist Gon and Killua face together, with the chimera ant royal guard having unceremoniously murdered their mentor, Kite. The first time, Killua has to make the awful decision to leave Kite behind to save Gon, his humanity blazing outwards as he chooses the lesser of two evils.
The second time is after Gon believes he has killed Neferpitou, and Killua has to push him out of the way when Neferpitou's body is animated post-humously. Each time Killua shows the lengths he is willing to go to defend Gon, even with his own life if he must.
3 Almost Lost It: Sending Nanika Away
When Nanika, the entity that inhabits Alluka's body and enables her to grant wishes, has finished healing Gon, Killua chooses to try and send it away forever. The only issue is that Nanika has thoughts and feelings of its own, and it openly loves Killua and enjoys spending time with him.
Killua, through tears, begins yelling at Nanika to leave and never come back and it sadly complies. It was a huge lapse of judgment on Killua's behalf, and he quickly rectifies the situation when Alluka herself scolds him and reminds him of what really matters.
2 Showed It: Sparing & Saving Ikalgo
While in the NGL, Killua comes face to face with chimera ant commander Ikalgo, a small-statured octopus with a lot to prove. Not only does Killua spare Ikalgo's life, but he saves it, and from that moment on he doesn't just gain an ally, but a genuine friend.
Ikalgo and Killua form a close bond from that point onwards and Ikalgo takes care of and saves Killua's life too in return. It is perhaps one of the greatest displays of Killua's developing humanity and a culmination of everything he has learned.
1 Almost Lost It: Threatening Gido, Riehlvelt, & Sadaso
During the Heavens Arena arc, Killua comes across three competitors who like to fight less than fair, and he considers more than once ending their lives immediately. While he eventually chooses to go with a more humane route, his attitude throughout indicates how close he was to falling off the edge.
Killua comments that killing them would be far easier, and his intimidation of Sadaso shows that he certainly wasn't opposed to finishing them all off the easy way.