To all the people saying Gon overreacted, please read this apforums post to understand what was going on through Gon's head.

I think that Gon's transformation was extremely well written and in line with the character. How about I do a recap and, perhaps, put things into perspective?

You know how Nen is strengthened by creating self-imposed conditions and taking vows, correct? It's why Kurapika's "Chain Jail" ability is so powerful--he will only use it on the Phantom Troupe and his own life is the toll for breaking said condition. I should mention that Nen is also strengthened by emotional pull.

Now a few things happened to Gon during the Chimera Ant arc. After losing to Knuckle (and thereby losing the right to enter NGL), he was forced to enter a state of Zetsu for a month. Readers have known since the Hunter Exam arc that Gon's greatest source of frustration is when he's forced to confront his own weakness, as he is extremely prideful and confident. During the Greed Island arc, Gon forsakes an easy victory against Genthru because he wants to prove to himself that he's strong--willingly sacrificing his own body just to get in a good hit. I digress, but for the sake of the argument.

After losing to Knuckle, Gon is shown his own weakness again. He sheds tears for the same reason he did after losing to Hisoka. Gon saw himself as strong enough to stand on his own, but is bluntly shown a reality he obviously can't stand. Gon isn't strong enough to rescue Kite--whom he sees as an older brother.

Nearing the end of his auraless month, Gon reunites with Kite...in a surprising manner. Even though Killua already had come to the conclusion that Kite had died, Gon held on because it's in his nature to be optimistic. He's still very much a child. This encounter changes him greatly for the rest of the arc.

The next chapter, Morel makes mention of how Gon has been compressing his anger in order to unleash it upon his enemy. Gon's learned a new technique: by forcefully entering Zetsu prior to attacking, his power explodes when he finally decides to attack.

Fast forward to the day of the national rally in East Gorteau. A quick digression. The Extermination Team still cannot figure out the reason as to why Neferpitou dispelled his En the day Knov broke into the East Gorteau Palace. They deduce that the King must've been injured by himself, but can't think of a reason as to why he would. Gon's answer is a bit surprising: "When...you can't forgive yourself."

Time's up. As the Extermination Team prepares to sprint, Killua looks over to Gon and sees his eyes darken. Symbolic of entering Zetsu--only, this time, there's something darker about them.

In front of his foe, Gon is put through the emotional wringer. Killua gets Gon to bottle his anger again, but it's clearly still there.

Eventually, we reach the climax. Kite is dead. And all of that anger Gon had directed towards Pitou gets redirected toward himself: he believes that he killed Kite. His emotions stretched to their limit, Gon makes the decision. He consumes all of his aura in a last ditch-effort: his condition and vow.

Gon utterly destroys Neferpitou. Killua rushes to his side and is horrified--what did his friend need to pledge in order to take such a form? Gon's answer isn't revealed until Pitou's corpse rips off his arm. "Finally...I'm the same as Kite was...The same as that time..."

When do you hurt yourself? "When you can't forgive yourself." It was a suicide move. Gon couldn't forgive himself for allowing Kite to die. So he sacrificed himself for this very reason. A conscious decision made when a kind boy feels guilty for the loss of a friend. He became the strongest ever--but only after feeling like the weakest. Gon's adult form was a projection of the person he wished he was: all-powerful. Strong enough to defeat his enemy. Strong enough to protect his friends.

So we basically learn that Gon doesn't handle loss well. But death is intrinsic to the natural world...especially intrinsic to the world of Hunters, whom, by profession, are often dealers of life and death. There's sure to be more of it in the manga, and I can't imagine how Gon would react if he lost someone even closer to him... like Ging, or Killua.