10 But Amasa did not see danger from the sword which was now in Joab's left hand, and Joab put it through his stomach so that his inside came out on to the earth, and he did not give him another blow. So Joab and his brother Abishai went on after Sheba, the son of Bichri. 11 And one of Joab's young men, taking his place at Amasa's side, said, Whoever is for Joab and for David, let him go after Joab! 12 And Amasa was stretched out in a pool of blood in the middle of the highway. And when the man saw that all the people were stopping, he took Amasa out of the highway and put him in a field, with a cloth over him, when he saw that everyone who went by came to a stop. 13 When he had been taken off the road, all the people went on after Joab in search of 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.