2 Chronicles 21:1-4
Jehoram kills off his own brothers to make sure that they do not usurp the throne. If we give him the benefit of the doubt, from a carnal standpoint, he may have had good reason to do what he did, because his brothers may have been indicating that they were already plotting to overthrow him out of jealousy. Perhaps they thought that they were every bit as good as Jehoram, and that they should sit on the throne instead. Jehoram, though, had more power and beat them to the punch, putting them to death before they assassinated him.
The background for this event begins in II Chronicles 18:1, where it innocently says, "Jehoshaphat had riches and honor in abundance, and by marriage he allied himself with Ahab." Ahab was possibly the most wicked king that Israel ever had, and his wife was the infamous Jezebel. How were relations cemented between Jehoshaphat and Ahab? They arranged a marriage! Jehoshaphat's son, Jehoram, married Ahab and Jezebel's daughter, Athaliah. This was a common way of making an alliance in those days. They became blood relatives.
John W. Ritenbaugh
Why Three Kings Are Missing From Matthew 1