Gon and Killua acknowledged that Greed Island is a game where it is implicit that you will be putting your life on the line. When you come into that game, you expect that you will be confronted with life or death situations. Killua contrasted that to the assassinations he performed in which his victims were not expecting to die or willing to put their lives on the line. Also, take into account Gon's attitude towards people trying to kill him. He does not hold it against his opponent. Gon's only anger issues with Genthru in that fight was that Genthru refused to use his ability initially, and that he went back on his word. Otherwise, his attitude towards Genthru would be more like "Well, if we fight, one of us may die. This is the nature of fighting, no hard feelings. But in the case I win yet you live, I will heal you. that's just who I am."

Also, last time I watched this, I didn't realize how close Gon came to winning that fight without resorting to the plan. Genthru got lucky in that he tripped over the rock and avoided the attack.