10 But Ama'sa did not observe the sword which was in Jo'ab's hand; so Jo'ab struck him with it in the body, and shed his bowels to the ground, without striking a second blow; and he died. Then Jo'ab and Abi'shai his brother pursued Sheba the son of Bichri.
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.