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 20:10-13
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.