Gon's transformation is the result of a "Vow" so extreme it requires he sacrifice his natural talent, life force, and future.
Killua reaches the scene only to find a transformed Gon standing over Pitou's corpse. Hunter X Hunter 2011 Episode 131
After realizing Pitou cannot revive Kite because he is truly dead, Gon's denial shatters. Overwhelmed by grief and rage, he makes a Nen contract to gain "all the power he will ever have," aging his body into a matured, monstrously powerful form. Gon's transformation is the result of a "Vow"
Gon's actions in this episode are a dark mirror of Kite's lessons. He selectively remembers Kite’s advice to "go for the head" as he brutally executes Pitou. Hunter X Hunter 2011 Episode 131
The final stage of grief (Acceptance) twisted into a self-destructive "Restriction and Pledge". Plot Summary