I don't entierly remember the whole thing, but weren't they just killing other players? and obviously they were murderes on the outside world, but that doesn't count.

did they kill people outside of the game world? I can't remember. even if they did, is that against the "rules" of the game?

he did the same with scisors-eat-girls-hair dude, he was also a murderer (and not only inside the game), but Gon liked him and let him live.

did Gon even ever kill anybody? maybe one of the ants? he was pretty pissed at Ilumi, but I don't think he was angry at Hisoka (also, clearly a murderer) or at any of the spider brigade. it's just like he said, they all played a game, and the strongest player gets to choose how the game goes. if the bomber is the strongest player and he wants the game to be played violently, it's within his rights as the strongest animal in the jungle. but once Gon takes the position, he decides the rules, and he doesn't want to kill Genjuro.

it doesn't matter that genjuro hurt him (remember hanzo?), or that he killed other people. it wasn't personal for them, so why should it be perssonal to him?