A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Chronicles 21:1
ch2 21:1JEHORAM SUCCEEDS JEHOSHAPHAT. (Ch2 21:1-4)
Jehoshaphat slept with his fathers . . . Jehoram . . . reigned--The late king left seven sons; two of them are in our version named Azariah; but in the Hebrew they appear considerably different, the one being spelt "Azariah," and the other "Azariahu." Though Jehoshaphat had made his family arrangements with prudent precaution, and while he divided the functions of royalty in his lifetime (compare Kg2 8:16), as well as fixed the succession to the throne in his oldest son, he appointed each of the others to the government of a fenced city, thus providing them with an honorable independence. But this good intentions were frustrated; for no sooner did Jehoram find himself in the sole possession of sovereign power than, from jealousy, or on account of their connections, he murdered all his brothers, together with some leading influential persons who, he suspected, were attached to their interest, or would avenge their deaths. Similar tragedies have been sadly frequent in Eastern courts, where the heir of the crown looks upon his brothers as his most formidable enemies, and is therefore tempted to secure his power by their death.
2 Chronicles 21:6
ch2 21:6HIS WICKED REIGN. (Ch2 21:5-7)
he walked . . . as did the house of Ahab, for he had the daughter of Ahab to wife--The precepts and examples of his excellent father were soon obliterated by his matrimonial alliance with a daughter of the royal house of Israel. Through the influence of Athaliah he abolished the worship of the Lord, and encouraged an introduction of all the corruptions prevalent in the sister kingdom. The divine vengeance was denounced against him, and would have utterly destroyed him and his house, had it not been for a tender regard to the promise made to David (Sa2 7:29; Kg2 8:19).
2 Chronicles 21:8
ch2 21:8EDOM AND LIBNAH REVOLT. (Ch2 21:8-17)
the Edomites revolted--That nation had been made dependent by David, and down to the time of Jehoshaphat was governed by a tributary ruler (Kg1 22:47; Kg2 3:9). But that king having been slain in an insurrection at home, his successor thought to ingratiate himself with his new subjects by raising the flag of independence [JOSEPHUS]. The attempt was defeated in the first instance by Jehoram, who possessed all the military establishments of his father; but being renewed unexpectedly, the Edomites succeeded in completely emancipating their country from the yoke of Judah (Gen 27:40). Libnah, which lay on the southern frontier and towards Edom, followed the example of that country.
2 Chronicles 21:12
ch2 21:12there came a writing to him from Elijah the prophet--That prophet's translation having taken place in the reign of Jehoshaphat [Kg2 2:11-12], we must conclude that the name of Elijah has, by the error of a transcriber, been put for that of Elisha.
2 Chronicles 21:13
ch2 21:13hast made Judah and the inhabitants of Jerusalem . . . like to the whoredoms of the house of Ahab--that is, introduced the superstitions and vices of PhÅnician idolatry (see on Deu 13:6-14). On this account, as well as for his unnatural cruelties, divine vengeance was denounced against him, which was soon after executed exactly as the prophet had foretold. A series of overwhelming calamities befell this wicked king; for in addition to the revolts already mentioned, two neighboring tribes (see Ch2 17:11) made hostile incursions on the southern and western portions of his kingdom. His country was ravaged, his capital taken, his palace plundered, his wives carried off, and all his children slain except the youngest. He himself was seized with an incurable dysentery, which, after subjecting him to the most painful suffering for the unusual period of two years, carried him off, a monument of the divine judgment. To complete his degradation, his death was unlamented, his burial unhonored by his subjects. This custom, similar to what obtained in Egypt, seems to have crept in among the Hebrews, of giving funeral honors to their kings, or withholding them, according to the good or bad characters of their reign.