4 Then the king said to Ama'sa, "Call the men of Judah together to me within three days, and be here yourself." 5 So Ama'sa went to summon Judah; but he delayed beyond the set time which had been appointed him. 6 And David said to Abi'shai, "Now Sheba the son of Bichri will do us more harm than Ab'salom; take your lord's servants and pursue him, lest he get himself fortified cities, and cause us trouble." 7 And there went out after Abi'shai, Jo'ab and the Cher'ethites and the Pel'ethites, and all the mighty men; they went out from Jerusalem to pursue Sheba the son of Bichri. 8 When they were at the great stone which is in Gibeon, Ama'sa came to meet them. Now Jo'ab was wearing a soldier's garment, and over it was a girdle with a sword in its sheath fastened upon his loins, and as he went forward it fell out. 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. 11 And one of Jo'ab's men took his stand by Ama'sa, and said, "Whoever favors Jo'ab, and whoever is for David, let him follow Jo'ab." 12 And Ama'sa lay wallowing in his blood in the highway. And any one who came by, seeing him, stopped; and when the man saw that all the people stopped, he carried Ama'sa out of the highway into the field, and threw a garment over him.
Matthew Henry's Commentary on 2 Samuel 20:4-12
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.