[Vaguely worded, but Spoilers Ahead for manga past chapter 300-something!]
I just thought it’d be fun if these two had to form a temporary truce in order to find the cure. Two flighty and dangerous Transformation users who, if not for Gon, would never end up dealing with each other alone (at least, not if Killua could help it). There’d be tense exchanges and silly awkwardness, over-analytical suspicion (on Killua’s part) and super perviness (on Hisoka’s part), and of course, danger all around. And secretly from the shadows, Illumi helping Hisoka be a troll. Overall, fun to be had by all~! ♠
With that said, I love how Togashi drew Hisoka’s face in chap 320, and just had to draw it somewhere myself, haha !