Commentaries
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.
2Sa 20:10-13. Amasa Is Slain.
10. smote him … in the fifth rib—the seat of the liver and bowels, where wounds are mortal.
struck him not again—that is, despatched him at the first blow.