9 And Jo'ab said to Ama'sa, "Is it well with you, my brother?" And Jo'ab took Ama'sa by the beard with his right hand to kiss him. 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 20:9-10
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.