10 Amasa didn't notice the sword in Joab's other hand. Joab stuck him in the belly and his guts spilled to the ground. A second blow wasn't needed; he was dead. Then Joab and his brother Abishai continued to chase Sheba son of Bicri.

Matthew Henry's Commentary on 2 Samuel 20:10

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.