Matthew Poole and continuators on 1 Samuel 28:2

2 And David said to Achish, Surely thou shalt know what thy servant can do . And Achish said to David, Therefore will I make thee keeper of mine head for ever.

He speaks ambiguously, as he did before, that Achish might understand him, as he did, of his acting for him against the Israelites; whereas he meant it of his acting for the Israelites against the Philistines, to which he was obliged both by Gods express command, and by his indelible and manifold Obligations to God, and to Gods People; and by his own manifest interest. Though it is likely he would have managed his Affais with all possible regard and care of Achish his person, to whom alone, upon the matter, David stood obliged, and not to the rest of the Philistines, who had an ill opinion of him, as we shall see.

For that Valour which I doubt not thou wiltst shew on my behalf.

i. e. Of my Body and Life, the Captain of my Life-Guard.