Hong Lung 18 Palms is rightly lauded as one of the most powerful martial arts ever described in Jin Yong's canon. Users of Hong Lung 18 Palms were typically among the most powerful martial artists of their era, and the skill was considered an elite martial art.
As far as its kill record, however, surprisingly...there's only one confirmed fatality from Hong Lung 18 Palms: Ah Chu at the hands of Kiu Fung in DGSD. Other than perhaps the anonymous fighters Kiu Fung killed at Tsui Yin Manor, nobody else was really ever *killed* by Hong Lung 18 Palms.
The reasons for this are various: first, Hong Lung 18 Palms was typically used against high-caliber (i.e. Greats-level) fighters who could match it with their own martial arts. Second, the users of Hong Lung 18 Palms were typically good-hearted, virtuous men who avoided killing unless it was absolutely necessary or warranted.
Still, it's surprising that this feared and lauded martial art has only one confirmed kill in the entire Jin Yong canon when the skill is featured in no fewer than four Jin Yong novels.