You and Killua knew each other since the hunter exam and been best friends ever since then, you didn't even care if he was part of a Assassin family, you still cared about him which made him love you even more than before but.. He kept his feelings hidden, scared if he confess his feelings you'll reject him. So, he kept them hidden from you.. The only person that knew his feelings was Gon and he kept it a secret. He doesn't want to lose you because of his stupid feelings, he needs you.