He that favoreth Joab - This speech, addressed to Amasa's followers as well as Joab's, shows very distinctly that the rivalry between Joab and Amasa, and David's purpose to make Amasa captain in Joab's room, were well known; and shows also the real reason why Joab killed Amasa. What is added, "and he that is for David," was intended to identify Joab's cause with David's, and also to insinuate that Amasa had not been loyal to David (2-Samuel 20:5 note).

He that favoureth Joab - As if he had said, There is now no other commander besides Joab; and Joab is steadily attached to David: let those therefore who are loyal follow Joab.

And one of Joab's men (h) stood by him, and said, He that favoureth Joab, and he that [is] for David, [let him go] after Joab.
(h) He stood by Amasa at Joab's appointment.

And one of Joab's men stood by him,.... By the body of Amasa; no doubt by the order of Joab, to satisfy the people as they came up, and reconcile them to this fact, and to exhort them not to stop, but to follow after Joab; for though Amasa their general was dead, Joab had taken the command of the army, and the pursuit was carried on with as much rigour as ever:
and said, he that favoureth Joab, and he that is for David, let him go after Joab: he that likes Joab should be general, and is in the interest of David, let him make no stay here, but follow after Joab; Joab and David are put together, as if their interests were the same; though there seems to be an indecency in placing Joab first.

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.

One of Joab's attendants remained standing by him (Amasa), no doubt at Joab's command, and said to the people who came thither, i.e., to the men of Judah who were collected together by Amasa (vid., 2-Samuel 20:4), "He that favoureth Joab, and he that (is) for David, let him (go) after Joab," i.e., follow him to battle against Sheba.

One - Left there on purpose to deliver the following message. Favoureth Joab - He that would have Joab to be general, rather than such a perfidious rebel as Amasa. For David - He that wisheth David good success against Sheba, and against all rebels.