5 So Amasa went to call together [the men of] Judah; but he delayed longer than the set time which he had appointed him. 6 And David said to Abishai, Now shall Sheba the son of Bichri do us more harm than did Absalom. Take thou thy lord's servants, and pursue after him, lest he get him fortified cities and escape our sight. 7 And there went out after him Joab's men, and the Cherethites, and the Pelethites, and all the mighty men; and they went out of Jerusalem, to pursue after Sheba the son of Bichri. 8 When they were at the great stone which is in Gibeon, Amasa came before them. And Joab was girded with his coat, his dress, and upon it was the girdle of the sword which was fastened on his loins in its sheath; and as he went forth it fell out. 9 And Joab said to Amasa, Art thou well, my brother? And Joab took Amasa by the beard with the right hand to kiss him. 10 And Amasa had taken no notice of the sword that was in Joab's hand: so he smote him with it in the belly and shed out his bowels to the ground, and struck him not again; and he died. And Joab and Abishai his brother pursued after Sheba the son of Bichri.
Matthew Henry's Commentary on 2 Samuel 20:5-10
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.