2 Chronicles 21:4 Commentary - English Annotations on the Holy Bible by Matthew Poole
He strengthened himself; he took courage and hardened his heart, as that word sometimes signifies.
Slew all his brethren with the sword; partly because they either did, or he knew that they would, oppose him in his wicked designs; and partly for his own security, lest his people, who, as he believed, would be highly exasperated with the execution of his counsels, should advance any of them to the throne, and depose him.
Of the princes of Israel; either,
1. Of Judah, here called Israel, or which See Poole "2Ch 21:2". Or,
2. The princes or chief men of Israel, properly so called; not the princes of all Israel, or of the several tribes thereof, but the chief of those Israelites who, out of love to God and to the true religion, had forsaken their estates and worthy advantages in the kingdom of Israel, and were now incorporated with the kingdom of Judah. These he especially struck at, either,
1. Because his wife instigated him thereunto, both to punish them for their revolt from her father, and to deter others from following their example. Or,
2. Because he justly and truly thought these would be most firm and constant to and zealous for that religion which he was resolved to oppose, being both by their conscience and interest obliged to it.
Consult other comments:
2 Chronicles 21:4 - Albert Barnes' Notes on the Bible
2 Chronicles 21:4 - Joseph Benson’s Commentary on the Old and New Testaments
2 Chronicles 21:4 - The Cambridge Bible for Schools and Colleges
2 Chronicles 21:4 - Adam Clarke's Commentary and Critical Notes on the Bible
2 Chronicles 21:4 - Companion Bible Notes, Appendices and Graphics
2 Chronicles 21:4 - Geneva Bible Notes
2 Chronicles 21:4 - John Gill's Exposition of the Entire Bible
2 Chronicles 21:4 - English Annotations on the Holy Bible by Matthew Poole
2 Chronicles 21:4 - Old and New Testaments Restoration Commentary
2 Chronicles 21:4 - John Trapp's Complete Commentary (Old and New Testaments)
English Annotations on the Holy Bible by Matthew Poole

Matthew Poole (1624–1679) wrote English Annotations on the Holy Bible, completing the chapters as far as Isaiah 58 before his death in 1679. The rest of the Annotations were completed by friends and colleagues among his Nonconformist brethren. The first printing of the completed edition was in 1685, 2 volumes folio, followed by editions in 1688, 1696 (with valuable chapter outlines added by the editors, Samuel Clark and Edward Veale), and the 4th and definitive edition in 1700, the basis of all others.