9 And Joab said to Amasa, "Is it well with you, my brother?" And Joab took Amasa by the beard with his right hand to kiss him. 10 But Amasa did not observe the sword that was in Joab's hand. So Joab struck him with it in the stomach and spilled his entrails to the ground without striking a second blow, and he died. Then Joab and Abishai his brother pursued Sheba the son of Bichri.
Matthew Henry's Commentary on 2 Samuel 20:9-10
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.