Killua Zoldyck is the secondary protagonist of the Hunter X Hunter series. He contrasts Gon in many ways, from his visual aesthetic to his morbid and mostly depressing backstory as the heir to a family of assassins.
For this reason, he is much more morally ambiguous than his frequently upbeat counterpart (save for the instance in which Freecss lost control). However, it begs whether or not Killua would be suited better in the role of an antagonist than that of a hero. He may even be a better antihero, at that. By analyzing every facet of his character, fans can determine whether this may be the case.
10 Hero - Talking Down Kurapika
Hellbent on revenge against the Phantom Troupe for what they had done to his family, Kurapika's initial plan was to slaughter every last one of the spider's members. While Gon and Killua initially ignored this ambition, it became more relevant at the beginning of the Yorknew arc.
While Freecss was obviously against such a foolhardy endeavor, it may have surprised one to know that Killua opposed it as well. Given his dark backstory (one which reflects Kurapika's own), he was in a better position to reason with him.
9 Villain - Failing Hunter Licensing Exam
At the end of the Hunter Exam, it was revealed to be designed to be only one of the remaining contestants eliminated. Unfortunately, this would be Killua ~ though not out of a lack of merit.
He was disqualified because he was manipulated into murdering one of the other contestants, explicitly going against Isaac Netero's minimal list of rules. This demonstrates that he is pliable to his family's will and may have disposed toward such depravity in the future if guided correctly.
8 Hero - Braving The NGL
Gon's quest to find his father led him into the hands of his capable associate, Kite. The man was currently exploring an area called the NGL, which had been rumored to have undergone some disturbance. Later, it would be revealed a chimera ant outbreak, resulting in the seasoned hunter's demise.
Though Gon had a selfish ambition in defeating Meruem's forces (specifically, to recover and/or avenge his friend), Killua was willing to support him out of allegiance and an understanding of what the monsters might do if they are unleashed on the world. It was a sterling example of his heroic qualities.
7 Villain - The Zoldyck Family
The Zoldyck Family has not exclusively been comprised of antagonists (including their efforts against Chrollo and Meruem, respectively). However, they are a looming threat that has yet to be dealt with or even fully addressed.
However, with Killua having rescued Alluka from their clutches and being effectively disassociated from the family, the only way to reopen the story and find a conclusion for them is if the "black sheep" returns to their ranks. This would entail that he is once again an assassin and therefore assumes an antagonistic role.
6 Hero - Saving Alluka
One of the most heartfelt interactions of the series was Killua's interactions with his younger sibling, Alluka. Their bubbly nature is often juxtaposed with the morbid "doppelganger" that lurks just beneath the surface, one which is capable of granting their brother the wishes he requests ~ at a cost.
While he initially was incentivized to liberate them so that they could heal the friend of his injuries, he would later resolve to protect them even after Gon had been restored. It demonstrated moral integrity, one that the Zoldyck family is often sorely lacking.
5 Villain - Murdering Johness
During the Hunter Exam, Killua was matched against a lumbering opponent named Johness. His adversary had a fierce reputation for murdering dozens of people. The longer he was able to hold the assassin in one place (including through breaking his body and making him unable to surrender), the lower his sentence became before he was emancipated.
Killua took this to the opposite extreme, ripping out his heart with a flex of his fingers. While Johness's death may have been justified because of his actions, Killua took smug satisfaction in ending his life.
4 Hero - Standing Up To Youpi
While Knuckle and Meleoron may have proved a decent temporary distraction against Youpi, it was one that the royal guard was quickly able to overcome. His power was so great that few aside from Netero himself could hope to check it.
Heroically, Killua stepped up to defy him, using his newfound godspeed powers to keep his hulking opponent on his toes. Without his invaluable assistance, it's possible that Youpi would have joined another fight and made it insurmountably more difficult for the Hunters' forces to win the day.
3 Villain - Stalking Gon On His Date
When Palm asked to go on a date with Gon as a condition for her assistance, the young boy readily agreed. Killua was understandably suspicious of this, deciding it would be best to stalk the two on their private date.
Not only would he do this without Freecss's consent or prior knowledge, but it also went against the terms outlined in the deal between the young hunter and his newfound "girlfriend." It would display an absence of trust that, if cultivated, might make Killua a tragic antagonist.
2 Hero - Moral
Oftentimes, there is an underlying moral within anime which is intended to translate into the real world. In the case of the Hunter X Hunter universe (and Killua specifically), people can change. This is evinced through his transformation from a hollow assassin to a beacon of life and hope.
If Killua became an antagonist, it would completely defy the underlying message behind his existence and deprive the story of perhaps its strongest and most coherent message.
1 Villain - Plot Potential
Having been with Gon since the very beginning, Killua and his friend have yet to engage in a serious fight against one another. This creates an open-ended question which many fans have pondered, who would win between the two of them if neither were holding back.
Such a dichotomy was explored in other anime such as My Hero Academia with Bakugo and Midoriya and Naruto, with Sasuke and the title character. If Killua reconsiders the path he had assumed in the canon series, it will create an opportunity for such reckoning to occur.