Jehoram became king when his father, Jehoshaphat, died. But Jehoram was not a good king like his father. He did not obey God like Jehoshaphat did. Instead, Jehoram paid attention to his wife, Athaliah. Remember that Athaliah was the daughter of Ahab and Jezebel. Athaliah taught Jehoram to worship false gods (2 Chronicles 21:11). She also influenced Jehoram to kill his brothers. Then Jehoram did not need to worry about his brothers trying to become king.
God was very angry with Jehoram. God caused the countries around Judah to rebel against Jehoram (2 Chronicles 21:8-10, 16-17). God also caused the prophet Elijah to write a letter to Jehoram. This letter told Jehoram that he would be punished with a terrible disease (2 Chronicles 21:12-15). Later, Jehoram died from that disease (2 Chronicles 21:18-20).
When Jehoram died, the people did not honor him. He had done many bad things. Why? Because he paid attention to Athaliah instead of paying attention to God.