Why did Bisky ask Killua to leave Gon? In Hunter x Hunter, Killua had to leave Gon because he wasn’t able to defeat Shoot, and Bisky told Killua his weakness and one day he would leave Gon to die. Later on, Killua removed the thing Illumi implanted into him, which changed him.

But I still don’t understand why Bisky would ask Killua to leave Gon?
Killua was joking. By saying to Gon about Bisky's promise, the only thing he meant was at that moment as he felt that Aluka needs him more than Gon, 'cause his family was hunting for Aluka. So he wants to devote his time to protect Aluka.