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. 11 And one of Joab's men took his stand by Amasa, and said, "Whoever favors Joab, and whoever is for David, let him follow Joab." 12 Amasa lay wallowing in his blood on the highway, and the man saw that all the people were stopping. Since he saw that all who came by him were stopping, he carried Amasa from the highway into a field, and threw a garment over him. 13 Once he was removed from the highway, all the people went on after Joab to pursue Sheba son of Bichri.
Matthew Henry's Commentary on 2 Samuel 20:10-13
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.