Joab was a son of Zeruiah, King David’s sister and was therefore one of David’s nephews.
“And Jesse begat his firstborn Eliab, and Abinadab the second, and Shimma the third, Nethaneel the fourth, Raddai the fifth, Ozem the sixth, David the seventh: Whose sisters were Zeruiah, and Abigail. And the sons of Zeruiah; Abishai, and Joab, and Asahel, three. And Abigail bare Amasa: and the father of Amasa was Jether the Ishmeelite.” 1 Chronicles 2:13–17
Joab’s brothers were two of David’s warriors, Abishai and Asahel. Joab was positioned as commander of David’s armies because of his victory over the Jebusites, resulting in the possession of the city of Jerusalem. It was through this victory that Jerusalem became “the city of David” (1 Chronicles 11:4–9).
Joab fought and won many battles for the king, but his personal lack of self-control was problematic. In a war against the forces of Ish-Bosheth, Joab’s brother Asahel was killed by Abner, the commander of Ish-Bosheth’s armies. Joab was furious and pursued Abner to kill him, but Abner escaped (2 Samuel 2:12–32). Later, after Abner swore allegiance to David, Joab lost his temper and his desire to avenge his brother’s blood drove him to deceive and murder Abner.
“And when Joab was come out from David, he sent messengers after Abner, which brought him again from the well of Sirah: but David knew it not. And when Abner was returned to Hebron, Joab took him aside in the gate to speak with him quietly, and smote him there under the fifth rib, that he died, for the blood of Asahel his brother.” 2 Samuel 3:26–27
This action deeply grieved David, but the king felt unable to bring justice against Joab (2 Samuel 3:38-39). Instead, David pronounced a curse over Joab and his future descendants:
“And afterward when David heard it, he said, I and my kingdom are guiltless before the Lord for ever from the blood of Abner the son of Ner: Let it rest on the head of Joab, and on all his father's house; and let there not fail from the house of Joab one that hath an issue, or that is a leper, or that leaneth on a staff, or that falleth on the sword, or that lacketh bread. So Joab, and Abishai his brother slew Abner, because he had slain their brother Asahel at Gibeon in the battle.” 2 Samuel 3:28–30
As the commander of David’s armies, Joab was provided many victories by God, but Joab caused much grief to the king and to Israel. His anger and perhaps the power of his position drove him to poor decisions at times. In addition to his murder of Abner, Joab killed his own cousin, Amasa—and his betrayal was Judas-style, accompanied by a kiss:
“And Joab said to Amasa, Art thou in health, my brother? And Joab took Amasa by the beard with the right hand to kiss him. But Amasa took no heed to the sword that was in Joab's hand: so he smote him therewith in the fifth rib, and shed out his bowels to the ground, and struck him not again; and he died. So Joab and Abishai his brother pursued after Sheba the son of Bichri.” 2 Samuel 20:9–10
Joab disobeyed King David’s command to spare Absalom’s life, himself striking Absalom with three javelins (2 Samuel 18). David mourned the death of his son Absalom, a response that was sternly reprimanded by Joab (2 Samuel 19:1–8). It was also Joab who in accordance with David’s command, placed Uriah the Hittite at the front of the battle to be killed, so that David could feel justified in marrying Uriah’s widow (2 Samuel 11).
Joab, for all his faults, was a capable man of war and valiant on the battlefield. And he was loyal to David for almost four decades. Joab also counseled David when David sinfully desired to take a census; if David had heeded Joab’s advice, he could have spared his nation the plague that befell Israel (2 Samuel 24).
When David was on his death bed, Joab conspired with Adonijah to install Adonijah as the next king, instead of Solomon (1 Kings 1). This action, plus Joab’s other rash decisions, vengeful murders, and inability to take certain important orders, finally drove David over the edge. David commanded Solomon to ensure Joab’s execution, an act that was carried out by Benaniah as Joab was clinging to the horns of the altar in hopes of finding clemency (1 Kings 2:5–6, 28–34).