nope, not sudden change at all in GON (with a k), kaito: his friend, his father's close friend and student, and gon's mentor (who saved his life as one of his first memories) was transformed into some horrific beast BY Pitou, the one he's so angry with.
and no massive boost in power yet, they said how Pitou could slash Gon's throat with one strike, or atleast she THOUGHT she could.
Gon's just pissed and sick of waiting, while his friend/mentor's life is on the line.
I also didnt see where he was per say "willing" to kill a girl so much as just "willing" to make a threat, his character shows he wouldnt do it, Im almost 110%, that is just a threat to get Pitou to hurry, AND as of now, he just assumes the girl is another enemy as there has been no indication to him otherwise.
I do think Pitou's emotional change is a little extreme, but there seems to be big emotional changes going on with all the ants so I'm assuming this has to do with the central theme at this part in the arc.
she IS funny though, all confused, and innocent ^ ^
I think Gon's boldness and anger is, like you guys said 4 years ago, BAMF lol.