Lest they should tell on us; that the tidings of this action against this people (who were, it seems, either tributaries to or confederates with Achish) might neither come quickly nor certainly to Achish's court; which he might the rather promise himself, because Achish and all his men were now busily employed in their warlike preparations against the Israelites; and if any flying rumour came thither, he thought by his interest and artifices he could easily discredit and dash it. Besides, the consideration of God's curse denounced against the people whom he had now destroyed, and of God's particular promises made to him, and of his special providence which he constantly experienced watching over him, made him more secure and confident in this and in many other hazardous attempts.
Related Commentaries of 1 Samuel 27:11
1 Samuel 27:11
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.