Each of Jin Yong's protagonists killed another human being during the course of his wulin adventures. Let's tally up when each hero scored his first kill.
Kiu Fung (DGSD): He was just a boy of ten when he murdered a doctor who cruelly refused to treat Kiu Fung's ill adopted father because the Kius were too poor to pay for the doctor's services.
Deun Yu (DGSD): I think he scored his first kill when he threw some rocks at Madame Wong's gang of hags when they pursued him and Muk Yeun Ching, causing one of the hags to fall down a sheer cliff to her death.
Hui Juk (DGSD): On errant information from Soo Sing Ho, accidentally killed two of his Shaolin martial uncles while trying to use his new inner power (from Mo Ngai Tze) to cure their poisoning by Ding Chun Chou.
Gwok Jing (LOCH): At the age of six, stabbed Chan Yeun Fung to death in Mongolia.
Yeung Gor (ROCH): I don't think he killed anyone until he was around eighteen years old...a few Mongolian soldiers he found terrorizing some peasants, maybe?
Cheung Mo Gei (HSDS): I don't know if this counts, but it's probably when he tricked those guys who were planning to eat him and Yeung But Fui into drinking that broth that he had spiked with poisonous mushrooms. I don't think he killed anyone with his martial arts skills until very late in HSDS, when he offed some of Sing Kwun's flunkies near Shaolin.
Ling Wu Chung (SPW): Don't know. Fill it in here, guys.
Wai Siu Bo (DOMD): Don't remember. I have a feeling it was fairly early in his adventures.
I don't know much about other Jin Yong protagonists such as Sek Por Tin and Yeun Sing Chi, so you guys fill it in.