Adam Clarke on 1 Samuel 29:8

David said - what have I done? - Dr. Chandler and others may say what they will to make David act a consistent part in this business; but it is most evident, whatever his intentions might be as to the part he was to take in the approaching battle, he did intend to persuade Achish that he would fight against Israel; and affects to feel his reputation injured by not being permitted on this occasion to show his fidelity to the king of Gath.

It was in the order of God's gracious providence that the Philistine lords refused to let David go with them to this battle. Had he gone, he had his choice of two sins - First, If he had fought for the Philistines, he would have fought against God and his country. Secondly If he had in the battle gone over to the Israelites, he would have deceived and become a traitor to the hospitable Achish. God, therefore, so ordered it in his mercy that he was not permitted to go to a battle in which he was sure to be disgraced, whatever side he took, or with what success soever he might be crowned.