John Gill Bible Commentary

Now when Jehoram was risen up to the kingdom of his father,.... Succeeded him in it, and reigned alone, for he had reigned with his father some years before his death; see Gill on Kg2 8:16, he strengthened himself; in the kingdom, by some means or another closely attaching the greater part of the princes, and people of the land, unto him, when he thought himself well settled and established on the throne; though some understand it of making strong, or hardening his heart to do what is next mentioned: and slew all his brethren with the sword; either to get their riches into his hands, or lest, being religious princes, they should oppose his restoring idolatry, and for that reason the people should depose him, and set up one of them; and therefore he dispatched them out of the way to secure himself, and carry his point: and divers also of the princes of Israel; such who had removed out of the ten tribes, for the sake of religion, and therefore would never agree to the introduction of idolatry among them, for which reason Jehoram slew them; and perhaps they might express their disapprobation and abhorrence of his murder of his brethren.