About Gon forgiving him... Gon is the one with the twisted personality. His sense of morality is just weird, let's say. He lets the murderer that eats women's hair in Greed Island go because "he trained him", says that he was fine with the Phantom Troupe because he thought they were just incapable of feeling pity and sadness (that changed later), he actually liked Tonpa in the Hunter Exam: a dude who was to their faces sabotaging their chances to pass and risking their lives... So yeah, if Killua just tells him that he's done killing, he would just accept it.
Any other people who could judge Killua (like Bisky or other hunters) have probably killed, or committed other crimes, themselves (exceptions of course). ")
Cool to chat here =) Hunter x Hunter is my favorite series of all time; sorry if I got annoying at some point.