We find here,
I. That Jehoshaphat was a very careful indulgent father to Jehoram. He had many sons, who are here named (Ch2 21:2), and it is said (Ch2 21:13) that they were better than Jehoram, had a great deal more wisdom and virtue, and lived up to their education, which he went counter to. They were very hopeful, and any of them more fit for the crown than he; and yet, because he was the first-born (Ch2 21:3), his father secured the kingdom to him, and portioned his brethren and disposed of them so as that they would be easy and give him no disturbance; as Abraham, when he made Isaac his heir, dismissed his other children with gifts. Herein Jehoshaphat was very kind and fair to his son, which might have obliged him to be respectful to him, and tread in the steps of so good a father. But it is no new thing for the children that have been most indulged by their parents to be least dutiful to them. Whether in doing this he acted wisely and well for his people, and was just to them, I cannot say. His birthright entitled him to a double portion of his father's estate, Deu 21:17. But if he appeared utterly unfit for government (the end of which is the good of the people), and likely to undo all that his father had done, it would have been better perhaps to have set him aside, and taken the next that was hopeful, and not inclined as he was to idolatry. Power is a sacred thing, with which men may either do much good or much hurt; and therefore Detur digniori - Let him that deserves it have it. Salus populi suprema lex - The security of the people is the first consideration.
II. That Jehoram was a most barbarous brother to his father's sons. As soon as he had settled himself in the throne he slew all his brethren with the sword, either by false accusation, under colour of law, or rather by assassination. By some wicked hand or other he got them all murdered, pretending (it is likely) that he could not think himself safe in the government till they were taken out of the way. Those that mean ill themselves are commonly, without cause, jealous of those about them. The wicked fear where no fear is, or pretend to do so, in order to conceal their malice. Jehoram, it is likely, hated his brethren and slew them for the same reason that Cain hated Abel and slew him, because their piety condemned his impiety and won them that esteem with the people which he had lost. With them he slew divers of the princes of Israel, who adhered to them, or were likely to avenge their death. The princes of Judah, those who had taught the good knowledge of the Lord (Ch2 17:7), are here called princes of Israel, as before fathers of Israel (Ch2 19:8), because they were Israelites indeed, men of integrity. The sword which the good father had put into their hands this wicked son sheathed in their bowels. Woe unto him that thus foundeth a kingdom in blood (Hab 2:12); it will prove a foundation that will sink the superstructure.
III. That Jehoram was a most wicked king, who corrupted and debauched his kingdom, and ruined the reformation that his good father and grandfather had carried on: He walked in the way of the house of Ahab (Ch2 21:6), made high places, which the people were of themselves too forward to make, and did his utmost to set up idolatry again, Ch2 21:11. 1. As for the inhabitants of Jerusalem, where he kept his court, he easily drew them into his spiritual whoredom: He caused them to commit fornication, seducing them to eat things sacrificed to idols, Rev 2:20. 2. The country people seem to have been brought to it with more difficulty; but those that would not be corrupted by flatteries were driven by force to partake in his abominable idolatries: He compelled Judah thereto. He used that power for the destruction of the church which was given him for the edification of it.
IV. That when he forsook God and his worship his subjects withdrew from their allegiance to him. 1. Some of the provinces abroad that were tributaries to him did so. The Edomites revolted (Ch2 21:8), and, though he chastised them (Ch2 21:9), yet he could not reduce them, Ch2 21:10. 2. One of the cities of his own kingdom did so. Libnah revolted (Ch2 21:10) and set up for a free state, as of old it had a king of its own, Jos 12:15. And the reason is here given, not only why God permitted it, but why they did it; they shook off his government because he had forsaken the Lord God of his fathers, had become an idolater and a worshipper of false gods, and they could not continue subject to him without some danger of being themselves also drawn away from God and their duty. While he adhered to God they adhered to him; but, when he cast God off, they cast him off. Whether this reason will justify them in their revolt of no, it will justify God's providence which ordered it so.
V. That yet God was tender of his covenant with the house of David, and therefore would not destroy the royal family, though it was so wretchedly corrupted and degenerated, Ch2 21:7. These things we had before, Kg2 8:19-22. The tenour of the covenant was that David's seed should be visited for their transgressions, but the covenant should never be broken, Psa 89:30, etc.

The kingdom gave he to Jehoram - He made him co-partner with himself in the kingdom about three years before his death; so that he reigned only five years after the death of his father Jehoshaphat. See the notes on Kg2 8:16, etc.; and on the same, Ch2 1:17, where an attempt is made to settle this disturbed chronology.

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.