5 So Amasa went to call the men of Judah together; but he stayed longer than the set time which he had appointed him. 6 David said to Abishai, “Now Sheba the son of Bichri will do us more harm than did Absalom. Take your lord’s servants, and pursue after him, lest he get himself fortified cities, and escape out of our sight.” 7 There went out after him Joab’s men, and the Cherethites and the Pelethites, and all the mighty men; and they went out of Jerusalem, to pursue after Sheba the son of Bichri. 8 When they were at the great stone which is in Gibeon, Amasa came to meet them. Joab was girded with his apparel of war that he had put on, and thereon was a sash with a sword fastened on his waist in its sheath; and as he went forth it fell out. 9 Joab said to Amasa, “Is it well with you, my brother?” Joab took Amasa by the beard with his right hand to kiss him. 10 But Amasa took no heed to the sword that was in Joab’s hand. So he struck him with it in the body, and shed out his bowels to the ground, and didn’t strike him again; and he died. Joab and Abishai his brother pursued after Sheba the son of Bichri.
Matthew Henry's Commentary on 2 Samuel 20:5-10
Commentary on 2 Samuel 20:4-13
(Read 2 Samuel 20:4-13)
Joab barbarously murdered Amasa. The more plot there is in a sin, the worse it is. Joab contentedly sacrificed the interest both of the king and the kingdom to his personal revenge. But one would wonder with what face a murderer could pursue a traitor; and how, under such a load of guilt, he had courage to enter upon danger: his conscience was seared.