Through Ahaziah's1 visit to Joram, God brought about Ahaziah's downfall. When Ahaziah arrived, he went out with Joram to meet Jehu son of Nimshi, whom the LORD had anointed to destroy the house of Ahab.
He then went in search of Ahaziah, and his men captured him while he was hiding3 in Samaria. He was brought to Jehu and put to death. They buried him, for they said, "He was a son of Jehoshaphat, who sought4 the LORD with all his heart." So there was no one in the house of Ahaziah powerful enough to retain the kingdom.
When Athaliah the mother of Ahaziah saw that her son was dead, she proceeded to destroy the whole royal family of the house of Judah.
References for 2 Chronicles 22:10
But Jehosheba,a the daughter of King Jehoram, took Joash son of Ahaziah and stole him away from among the royal princes who were about to be murdered and put him and his nurse in a bedroom. Because Jehosheba,b the daughter of King Jehoram and wife of the priest Jehoiada, was Ahaziah's sister, she hid the child from Athaliah so she could not kill him.