Chronicles II 21:4

4 And Joram entered upon his kingdom, and strengthened himself, and slew all his brothers with the sword, and of the princes of Israel.

Chronicles II 21:4 Meaning and Commentary

2 Chronicles 21:4

Now when Jehoram was risen up to the kingdom of his father,
&c.] Succeeded him in it, and reigned alone, for he had reigned with his father some years before his death; (See Gill on 2 Kings 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.

Chronicles II 21:4 In-Context

2 And he had brothers, the six sons of Josaphat, Azarias, and Jeiel, and Zacharias, and Azarias, and Michael, and Zaphatias: all these the sons of Josaphat king of Juda.
3 And their father gave them many gifts, silver, and gold, and arms, together with fortified cities in Juda: but he gave the kingdom to Joram, for he the first-born.
4 And Joram entered upon his kingdom, and strengthened himself, and slew all his brothers with the sword, and of the princes of Israel.
5 When he was thirty and two years old, Joram succeeded to his kingdom, and he reigned eight years in Jerusalem.
6 And he walked in the way of the kings of Israel, as did the house of Achaab; for a daughter of Achaab was his wife: and he did that which was evil in the sight of the Lord:

The Brenton translation of the Septuagint is in the public domain.