Born into a family of very secretive and exceptionally deadly assassins, Killua was put through rigerous training all through his childhood. Eventually tiring of his family's over damanding ways, he stabbed his mother in the face, disowned the family, and left to become a Hunter. He met other hopefuls including Gon and befriended them while doing the tests to become a hunter. After a really long story about becoming a hunter, and a lot of other adventures...
Killua-san decided to try his hand at other things and has been in New Eden for some time. He has made an honest living while in New Eden but the old instincts of an assassin are still there.