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I'm currently watching the anime series Hunter x Hunter. I'm currently on the episode 30. But looking back, I get confused why exactly did Killua kill the opponent of Leorio.
Before the fight, his brother was seemingly hypnotizing him that he will need to step back if he know that his opponent is stronger than him, thus making him protecting Killua.
His brother also stated that a certain day will come that Killua will leave Gon to die. I don't get the point why such action will be done to Kill the opponent of Leorio.