4 Then the king told Amasa, "Mobilize the army of Judah within three days, and report back at that time." 5 So Amasa went out to notify Judah, but it took him longer than the time he had been given. 6 Then David said to Abishai, "Sheba son of Bicri is going to hurt us more than Absalom did. Quick, take my troops and chase after him before he gets into a fortified town where we can't reach him." 7 So Abishai and Joab, together with the king's bodyguard and all the mighty warriors, set out from Jerusalem to go after Sheba. 8 As they arrived at the great stone in Gibeon, Amasa met them. Joab was wearing his military tunic with a dagger strapped to his belt. As he stepped forward to greet Amasa, he slipped the dagger from its sheath. 9 "How are you, my cousin?" Joab said and took him by the beard with his right hand as though to kiss him. 10 Amasa didn't notice the dagger in his left hand, and Joab stabbed him in the stomach with it so that his insides gushed out onto the ground. Joab did not need to strike again, and Amasa soon died. Joab and his brother Abishai left him lying there and continued after Sheba. 11 One of Joab's young men shouted to Amasa's troops, "If you are for Joab and David, come and follow Joab." 12 But Amasa lay in his blood in the middle of the road, and Joab's man saw that everyone was stopping to stare at him. So he pulled him off the road into a field and threw a cloak over him. 13 With Amasa's body out of the way, everyone went on with Joab to capture Sheba son of Bicri.
Matthew Henry's Commentary on 2 Samuel 20:4-13
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.