I'm pretty sure if Gon attacked Nef, she would have defended herself, causing Komugi to die, then she will kill the two brats in vengeance. Simply dying is not a choice, because that would have also caused Komugi to die, leaving Nef with nothing.

In the larger scheme of things, Killua made the right choice, since it delayed the guard and saved his friend's soul.

I really hope Nef doesn't turn traitor after this. I really like her and I hate those who go back on their words.