From the outside, Killua seems just to be the rich boy; cheeky, cheerful, has a sweet tooth, and full of mischievous ideas. However, his ruthlessness and aptitude in killing shows the other side of him-- deadly, violent and bloodthirsty.
Having been repeatedly tortured since birth, Killua has been conditioned to possess extreme tolerance for poison, electricity and pain. He possesses extraordinary agility and strength that makes Killua a one man killing machine-- not one to be trifled with, as many of his adversaries had witnessed.
However, until recently, his greatest flaw was to fear those who are more powerful than him, one of whom is his elder brother, Illumi. Killua had been trained as an assassin by his brother to be extremely cautious that he only engages in combat provided that victory is absolutely certain. This impeded him as a Hunter, as Biscuit stated more than once that victory is never certain, and that depending on the circumstances, a person has the potential to defeat a stronger opponent.
Killua struggled with these conflicting ideas during the latter half of the series, as he started to encounter stronger opponents. However, his brother's teachings remained firmly entrenched in his mind, to the point that Biscuit prophesied that he would one day abandon Gon because of this. Biscuit gave Killua an ultimatum: Defeat Shoot to claim the right to go with Gon and fight the Chimera Ants. Killua lost, and resolved to leave after Gon had recovered from his fight, protecting him in the meantime.
Finally he had to face Biscuit's prophecy when he encountered a Chimera ant while Gon (who couldn't use nen at the time) wasn't far off and had no way to defend himself. Killua made an attempt to save Gon. While fighting with the Chimera Ant, Killua is also battling his inner-self. Preoccupied, Killua is losing to the chimera ant. At the last moment, Killua realized that Illumi had implanted a needle in his brain way back in the first Hunter Exam. This needle was responsible for forcing Killua to retreat and jailed himself on his comfort zone whenever any situation would potentially endanger his life. Upon extracting the needle, Killua was freed from this restraint and gained full control of his subconscious to make his own decision whether or not to flee whenever there is a need to fight stronger opponents.
In the manga, Killua is much more easygoing and sociable. In the anime, he is noticibly more sullen, and seems to be more frightened of his assasin side.
At first part of the series, Killua does not usually employ or use any weapons in combat. Instead, he relies on his assassin-type abilities, his hands that have been modified that changes into razor sharp claws, delivering swift and fatal attacks that can rip out beating hearts or behead opponents in a split second.
During the Greed Island story arc, Killua started experimenting with using multiple yo-yo's, made of special alloy, as weapons. The yo-yo's are extremely durable and heavy, weighing 50 kilograms each. They can be used in conjunction with Killua's electrical Nen abilities. Killua is reluctant to use the yo-yos in combat thus using only his raw physical abilities in order for his opponent to know as little about his combat abilities as possible.
 Technique of the Dark Step
A high-level assassin skill mastered by Killua, it involves creating the illusion that one is multiplying, and combined with the total absence of noise, makes an attack performed together with this technique hard to counter.
 Nen Abilities
As a Transmuter, Killua's most efficient in changing the form of his nen. As such, Killua developed several electricity based nen techniques. This feat was both inspired and made possible by the years of electric shocks he received as a child, both as training against torture and as punishment from his older brother Milluki.
Nen Ability: Lightning Palm (イズツシ 「雷掌」), Izutsushi
The first and most basic ability Killua developed. As the name suggests, Killua is able to shock an opponent by touching them.
Nen Ability: Thunderbolt (ナルカミ 「落雷」), Narukami
A natural evolution of his Lightning Palm ability, Killua is also able to project a large bolt of lightning from his hands, striking an opponent with electricity from a distance. The Japanese name of the ability, Narukami, comes from the famous Kabuki play, Narukami Fudô Kitayama Zakura, in which the character Narukami transforms into a thunder god at the end of the fourth act.
Nen Ability: Kanmuru (カンムル 「神速」), lit. "Godspeed"
Developed by Killua during his fight against the Ortho Siblings, Killua's Kanmuru ability allows him to react and move his body at speeds that would normally be impossible. This ability changes Killua's aura into a field capable of transmitting all electrical impulses from his brain in a straight line directly to the part of the body he wants to use, completely bypassing his nervous system. It is made up of two parts: Raikō Sekka, the ability to move his body according to his own will, and Shippū Jinrai, the ability to sense his opponents movements and have his body react automatically.