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.
11-13. He that favoureth Joab, and he that is for David, let him go after Joab—It is a striking proof of Joab's unrivalled influence over the army, that with this villainous murder perpetrated before their eyes they unanimously followed him as their leader in pursuit of Sheba. A soldier conjoined his name with David's, and such a magic spell was in the word "Joab," that all the people "went on"—Amasa's men as well as the rest. The conjunction of these two names is very significant. It shows that the one could not afford to do without the other—neither Joab to rebel against David, nor David to get rid of Joab, though hating him.