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Destiny or Coincidence.
Kurapika's Friend, Pyro died.
Leorio's Friend, Pietro died.
Killua(with the help of Alluka) saved Gon from dying
So I think there will be a major event that would make one or many from the four
(and most probably Killua) will be in the verge of dying.
And I still don't know if he will die or not.
Interesting point. Though I think it's more possible that it's a reason for them to be so friendly with each other (even though these strings are not visible to them as they are to us, readers) than a plot-device to make the story progress.
Well, this is a possibility that we can't ignore.
Still it's true that Gon and Killua already went through desperate situations, and Gon already risked to lose his life, as well as Killua back to the Chimera ants arc.