As an angel of God, in whom nothing is blameworthy. Or it may be used to express David's great wisdom (as well as integrity); as 2 Samuel 14:17, 2 Samuel 19:27. The heathens acknowledged good spirits, which also they worshipped as an inferior sort of deities, who were messengers and ministers to the supreme God; only Achish had learned the title of angels from the Israelites his neighbours, and especially from David's conversation.
Related Commentaries of 1 Samuel 29:9
1 Samuel 29:9
9 And Achish answered and said to David, I know that thou art good in my sight, as an angel of God: notwithstanding the princes of the Philistines have said, He shall not go up with us to the battle.