Killua is a strategic genius, but ask yourself – if things go completely off the rails at the key moment, who would trust to make the right decision when there was no plan in place, Gon or Killua? I’d trust Gon, because he knows there’s a time to think and a time to trust your instincts. If Gon thinks too little sometimes, Kil certainly thinks too much – and I think it’s the vulnerability that prompted Shoot’s “fade away” comment (spotted, I think, because Shoot has the same worry about himself).
He’s correct in the specific instance of teaching Ikalgo that he’s wrong to assume the Royal Guard could never leave The King, but in the big picture, he’s wrong. Killua utters the damning words himself, to Octobro – “You can’t overthink something.” But you can, Killua – that’s the problem. Cut to this episode, and Knov’s hidden room in the final moments.
