5 So Amasa went to summon Judah; but he delayed beyond the set time that had been appointed him. 6 David said to Abishai, "Now Sheba son of Bichri will do us more harm than Absalom; take your lord's servants and pursue him, or he will find fortified cities for himself, and escape from us." 7 Joab's men went out after him, along with the Cherethites, the Pelethites, and all the warriors; they went out from Jerusalem to pursue Sheba son of Bichri. 8 When they were at the large 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 at his waist; as he went forward it fell out. 9 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 notice the sword in Joab's hand; Joab struck him in the belly so that his entrails poured out on the ground, and he died. He did not strike a second blow. Then Joab and his brother Abishai pursued Sheba 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.