5 So Amasa went to summon Judah, but he delayed beyond the set time that had been appointed him. 6 And David said to Abishai, "Now Sheba the son of Bichri will do us more harm than Absalom. Take your lord's servants and pursue him, lest he get himself to fortified cities and escape from us."[1] 7 And there went out after him Joab's men and the Cherethites and the Pelethites, and all the mighty men. They went out from Jerusalem to pursue Sheba the son of Bichri. 8 When they were at the great stone that is in Gibeon, Amasa came to meet them. Now Joab was wearing a soldier's garment, and over it was a belt with a sword in its sheath fastened on his thigh, and as he went forward it fell out. 9 And Joab said to Amasa, "Is it well with you, my brother?" And Joab took Amasa by the beard with his right hand to kiss him. 10 But Amasa did not observe the sword that was in Joab's hand. So Joab struck him with it in the stomach and spilled his entrails to the ground without striking a second blow, and he died. Then Joab and Abishai his brother pursued 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.