Kirk would not have received mercy because the Gorn was still under the assumption that Kirk’s people invaded his planet. All of the Gorn’s actions in the episode were about baiting and destroying what they perceived to be a threat to them. If the Gorn just attacked for no reason, or were instinctively just trying to kidnap the Cestus III personnel or the Enterprise crew in order to use them as breeding stock (which did not at all seem to be the case), then that would not show much intelligence or civilization on their part. But the circumstances as presented in the episode, and later info about them from DS9 shows that the Gorn are in fact a civilized intelligent race and more than just mindless xenomorphs.