Jamieson, Fausset and Brown on 2 Samuel 18:16

16. Joab blew the trumpet, ... and held back the people--Knowing that by the death of the usurper there was no occasion for further bloodshed, he put an end to the pursuit and thereby evinced the temperate policy of his conduct. However harsh and unfeeling to the king Joab may appear, there can be no doubt that he acted the part of a wise statesman in regarding the peace and welfare of the kingdom more than his master's private inclinations, which were opposed to strict justice as well as his own interests. Absalom deserved to die by the divine law (De 21:18, 21), as well as being an enemy to his king and country; and no time was more fitting than when he met that death in open battle.