2 Chronicles 21:13
But hast walked in the way of the kings of Israel, and hast made Judah and the inhabitants of Jerusalem to go a whoring, like to the whoredoms of the house of Ahab, and also hast slain thy brethren of thy father’s house, which were better than thyself:
The charge was sent in writing by Elijah to Jehoram king of Judah before Elijah was translated, because Judah was pursuing the same course that had brought the northern kingdom to the verge of ruin and could not be left unreproved.
Jehoram did not walk in the ways of Jehoshaphat and Asa but walked in the way of the kings of Israel in their evil ways. He had joined himself closely to Israel by marriage to the daughter of Ahab and Jezebel.
He followed after Baal like the house of Ahab. He made high places in the mountains of Judah and caused the inhabitants of Jerusalem to commit fornication and compelled Judah thereto. This making of Judah and Jerusalem to go a whoring like the whoredoms of the house of Ahab was idolatrous apostasy.
He also slew his brethren of his father's house, murdering his own brothers, who were better than himself.
These acts are presented as the reason for the coming judgment on Jehoram and his household described in the surrounding verses. God would smite his people, children, wives and goods with a great plague and smite him himself with great sickness. That was fulfilled when the Philistines and Arabians carried away his substance and his sons and wives except his youngest son, and when incurable disease of the bowels ended in his death after two years. The account of Jehoram's wrongdoing that brought that judgment places Ahaziah's reign after Jehoram's death and presents Ahaziah's conduct as continuing the influence of Ahab's house.
The wrongdoing belongs within the history of Ahab's house, whose destruction God commissioned Jehu to bring as judgment for the blood of the Lord's servants and prophets shed by Jezebel.