Achish considers David to be a good servant. In fact, in comparing him to an angel of God, he is drawing attention to his moral excellence, i.e. his ability to discern right from wrong and choose between two difficult ethical options (cf. 2 Samuel 14:17, 2 Samuel 19:27).1
9 And Achish answered David and said, “I know that you are as blameless in my sight as an angel of God. Nevertheless, the commanders of the Philistines have said, ‘He shall not go up with us to the battle.’