Charles Ellicott Commentary 1 Samuel 29:8

"And David said unto Achish, But what have I done? and what hast thou found in thy servant so long as I have been before thee unto this day, that I may not go and fight against the enemies of my lord the king?" — 1 Samuel 29:8 (ASV)

And David said to Achish, But what have I done? —David’s words have a ring of falseness in them; he never contemplated fighting in the ranks against Israel, and yet he speaks thus. The generous confidence of the chivalrous Achish is here in painful contrast with the dissimulation of the Israelite chieftain, David.

It has been suggested that these suspicions of his loyalty on the part of the Philistine leaders had been aroused by David deliberately, in order to bring about his dismissal from the army in the field. This is possible, for the situation in which David now finds himself was most embarrassing from every point of view.