Board Thread:Questions and Answers/@comment-26005711-20151226104253/@comment-29825745-20160901132429

Martialmaniac wrote: Hard to say. Illumi seems to be somewhat afraid of Hisoka. However, when Hisoka "threatened" Killua, Illumi was sure he could kill him easily. I still can't think of a reason why Hisoka has not started a fight to the death with him. For the moment, I'll go with Hisoka, but something keeps telling me it's really Illumi.

It seems to me that Illumi just said he'd kill Hisoka because he was 300.0% MAD at that point.