2 Samuel 18:13 - Joseph Benson’s Commentary on the Old and New Testaments

I should have wrought falsehood against my own life Not only have been false and disobedient to the king, but should have betrayed my own life, and therefore not have been true to myself. For there is no matter hid from the king This, as all other things, would certainly have come to the king's ear. Thou thyself wouldest have set thyself against me Thou wouldest have been my adversary and accuser, both because it would have been thy duty to be so, and to vindicate thyself from any blame in the matter. He knew the disposition of Joab so well, that he was sure that general would have been as forward as any one both to inform the king of what had been done, and to have had the person punished who did it, for disobeying his sovereign.