Why, you ask? There are 3 clues:
1) Chrollo vs Hisoka already happened, it would not be interesting to repeat an encounter, storywise.
2) Kurapika and Hisoka have no quarrel, even if the second is a pedophile murder hobo, they have a good history as business associates
3) I actually haven't more clues that point to Illumi being the finisher. But I seriously think nor Chrollo nor Kurapika will finish Hisoka, maybe a Zodiac will put the clown down, given that no one liked him even before he turned into a serious psychokiller qu'est-ce que c'est