10 But Amasa did not notice the sword that was in Joab's hand. And he struck him with it in the stomach, and his entrails poured out on the ground; and he did not strike him again. Thus he died. Then Joab and Abishai his brother pursued Sheba the son of Bichri. 11 Meanwhile one of Joab's men stood near Amasa, and said, "Whoever favors Joab and whoever is for David--follow Joab!" 12 But Amasa wallowed in his blood in the middle of the highway. And when the man saw that all the people stood still, he moved Amasa from the highway to the field and threw a garment over him, when he saw that everyone who came upon him halted. 13 When he was removed from the highway, all the people went on after Joab to pursue Sheba the 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.