1 Samuel 27:11
And David saved neither man nor woman alive, to bring tidings to Gath, saying, Lest they should tell on us, saying, So did David, and so will be his manner all the while he dwelleth in the country of the Philistines. And David saved neither man nor woman alive, to bring tidings to Gath, saying, Lest they should tell on us, saying, So did David, and so will be his manner all the while he dwelleth in the country of the Philistines.
David left neither man nor woman alive in the towns he raided, so that no survivor could bring tidings to Gath and report what he had really done. He destroyed every witness in order to keep his actions hidden from Achish. He then misled Achish about whom he had attacked, leading him to believe that David had turned against Israel. Achish was made to think David had been raiding his own people.
That deception strengthened the Philistines by giving them a false impression that Israel was weak. It also made them suppose that David was indifferent to the welfare of his own people. All this was done while he was dwelling in the country of the Philistines and living under their protection, and it was a poor return for the kindness shown to him. In taking this course he acted without first seeking the counsel of the Lord.