Title: The Sweetest Revenge
Spoilers: Early fic, probably around the end of the Hunter Exam.
Killua understood Kurapika better than Leorio or Gon. The others were too innocent to understand the intangible chains that bound Kurapika's soul.
He knew trying to talk him about it was pointless. Kurapica would have to discover for himself the bitterness vengance would bring. It would taste like ashes, unsatisfying and disgusting - and the blood on his hands would only steal his purity.
Killua knew, because he hated his family. Killua had rebelled, and never really regretted it. One day, he wanted Kurapica to realize the truth.
The best vengeance he'd ever find was in Leorio's smile, in Gon's laugh.