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. 11 One of Joab's soldiers took up his post over the body and called out, "Everyone who sides with Joab and supports David, follow Joab!" 12 Amasa was lying in a pool of blood in the middle of the road; the man realized that the whole army was going to stop and take a look, so he pulled Amasa's corpse off the road into the field and threw a blanket over him so it wouldn't collect spectators. 13 As soon as he'd gotten him off the road, the traffic flowed normally, following Joab in the chase after 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.