5 So Amasa went to get all the men of Judah together, but he took longer than the time David had given him. 6 And David said to Abishai, Sheba, the son of Bichri, will do us more damage than Absalom did; so take some of your lord's servants and go after him, before he makes himself safe in the walled towns, and gets away before our eyes. 7 So there went after Abishai, Joab and the Cherethites and the Pelethites and all the fighting-men; they went out of Jerusalem to overtake Sheba, the son of Bichri. 8 When they were at the great stone which is in Gibeon, Amasa came face to face with them. Now Joab had on his war-dress, and round him a band from which his sword was hanging in its cover; and while he was walking, it came out, falling to the earth. 9 And Joab said to Amasa, Is it well, my brother? And with his right hand he took him by the hair of his chin to give him a kiss. 10 But Amasa did not see danger from the sword which was now in Joab's left hand, and Joab put it through his stomach so that his inside came out on to the earth, and he did not give him another blow. So Joab and his brother Abishai went on 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.