And Absalom happened to meet the servants of David. Absalom was riding on his mule, and the mule went under the thick branches of a great oak, and his head caught fast in the oak, and he was suspended between heaven and earth, while the mule that was under him went on. And a certain man saw it and told Joab, “Behold, I saw Absalom hanging in an oak.” Joab said to the man who told him, “What, you saw him! Why then did you not strike him there to the ground? I would have been glad to give you ten pieces of silver and a belt.” But the man said to Joab, “Even if I felt in my hand the weight of a thousand pieces of silver, I would not reach out my hand against the king’s son, for in our hearing the king commanded you and Abishai and Ittai, ‘For my sake protect the young man Absalom.’ On the other hand, if I had dealt treacherously against his life (and there is nothing hidden from the king), then you yourself would have stood aloof.” Joab said, “I will not waste time like this with you.” And he took three javelins in his hand and thrust them into the heart of Absalom while he was still alive in the oak. And ten young men, Joab’s armor-bearers, surrounded Absalom and struck him and killed him. 2 Samuel 18:9-15.
Joab knows, beyond a shadow of a doubt, that David wants Absalom spared. But when Absalom gets caught in those oak branches, he takes it upon himself to do what needs to be done. All the people heard David command him to “Deal gently for my sake with the young man Absalom.” Joab’s own man reminds him when asked why Absalom is not hanging dead in that tree already.
Joab surely believes David’s judgment is clouded by his love for his son, and takes responsibility for doing what must be done. He knows that a living Absalom means constant conflict, rebellion and soldiers dying until either he or David is dead. Joab casts his lot with David, even though he will pay for it.
Joab is a killer. He’s not a model of godly behavior. He is more of a patriot, a man who serves what he believes is the national interest. He is not always right, as in the time he slew Abner, who had just come from making peace with David to unite Israel and Judah. But he is not always wrong either. And he is willing to die if that is what executing Absalom gets him. I believe Joab made the right call.