5 So Amasa went to call [the men of] Judah together; but he tarried longer than the set time which he had appointed him. 6 And David said to Abishai, Now will Sheba the son of Bichri do us more harm than did Absalom: take thou thy lord's servants, and pursue after him, lest he get him fortified cities, and escape out of our sight. 7 And 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. And Joab was girded with his apparel of war that he had put on, and thereon was a girdle with a sword fastened upon his loins in the sheath thereof; and as he went forth it fell out. 9 And Joab said to Amasa, Is it well with thee, my brother? And 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 smote him therewith in the body, and shed out his bowels to the ground, and struck him not again; and he died. And 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.