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.
I mean, at the beginning of the Greed Island arc, Bisky saw them walking together and said something like “nothing as precious as two boys’ friendship. I can’t wait to ruin it.” Or something like that. I don’t recall the exact quote, but tbh she could’ve been planning something like that from a while back.
She could have been setting it up but I highly doubt it. If she had wanted to do that she could have failed him in training. When she said that level 2 of emitting was launching himself in the air only using his arm. She could have kept him there and waved away Killua's Idea. Chances are, Killua has a job to do and so there are gonna be 2 separate arc's in the next season, kinda like the York New city auction, but they will end up merging again.