"Gon, you are light. Sometimes you shine so brightly that I must look away".
Killua was never the best science student; he was proficient enough to understand a few things. Thus, he had gotten into the habit of comparing Gon to light.
Throughout their entire friendship, Gon had shone like a star. Full of determination; full of hope and life. He contrasted Killua so much that it shouldn't have been possible for them to coexist peacefully. This thought nagged at Killua's mind throughout their adventures.
It was during the battle with Neferpitou (who was crouched on the ground) that Killua chose to remember a certain thought. While it was sort of silly for him to interpret his own comparisons so literally, Killua had an important revelation: even the brightest light cannot escape a black hole.
Gon was so driven by ambition, and he… he held no important purpose, other than to remain friends with Gon. Did their adventures create the black hole? Were Gon's ambitions too grand for him? Is that why he collapsed on himself? These questions pestered Killua as he waited for Nanika to finish healing Gon.
He was afraid he would never come to an answer. No longer were his queries going to be directed towards another; this time, he was questioning himself. It wouldn't be long until even the most vivid of stars inverted and became the darkest of black holes. The bigger you were, the harder you fell – and a part of Killua wanted Alluka and himself to be nowhere near Gon when this happened. Another part of him wanted to hug him close and keep him safe. For now, Killua settled for a goodbye, and a recurring epiphany.
"The brightest of stars create the most miserable of black holes."
Notes: I once read a short one-shot that compared Gon to a collapsing star. I wanted to elaborate on that idea.
I do not own HunterxHunter.