2 Samuel 18:13

I should have wrought falsehood against mine own life I should not only have disobeyed the king, but have been false to my own interest and forfeited my life. The Kthîbh reads his life, thus: Or if I had dealt deceitfully against his life, there is nothing hid, &c.: i.e., if I had treacherously slain him contrary to the king's command, it would certainly have come to the king's ears, and I should have been put to death. The Sept. has a different reading, connecting the first clause of 2 Samuel 18:13 with 2 Samuel 18:14, thus: "Take care of the young man Absalom for my sake, that ye do no harm against his life: and there is no matter hid," &c.

wouldest have set thyself against me Wouldest have taken part against me with the king. The man was well aware of Joab's unscrupulous character.