One of the reasons that David killed the men and women of the raiding parties was to make sure that there were no survivors who could challenge or contradict the story that he told Achish. For his deceit of Achish to be successful, he had to make sure there were no witnesses.
11 And David would leave neither man nor woman alive to bring news to Gath, thinking, “lest they should tell about us and say, ‘So David has done.’” Such was his custom all the while he lived in the country of the Philistines.