Killua was born as the middle child to a family of renowned assassins, the Zaoldyeck family. Showing great promise from birth, Killua has already mastered many killing techniques at a tender age, and is set to be one of the best assassins the family has produced. Bored and tired of killing, Killua unexpectedly rebelled against his family, injuring both his mother and brother, and ran away to join the Hunter exam. Meeting with Gon during the first obstacle, Killua saw Gon as a friendly competitor, and ended up becoming fast friends. The Zaoldyeck family are world-renowned assassins so rich and powerful that they are said to employ more than 200 people and live and own Kukuruu Mountain and the vast surrounding forests. They are so fearless that they have never hidden their whereabouts, and even tour buses take visitors to their "front gates" known as the Gates to the Underworld as those who enter never return.
Killua divulges that since his youth, he has been constant training, having been left in the Celestial Tower (a tournament arena) at the age of 6, and swimming in pools of electric eels being amongst his everyday trials.
From the grandmother, grandfather, father, mother, and five sons, all are assassins.