In Second Kings, chapter twelve, verse twenty, the conspiracy against Joash (Jehoash) occurred because he had rejected the Lord, begun to worship other gods, and killed the high priest Zechariah as in Second Chronicles twenty-four, verses seventeen through twenty-two and twenty-five, respectively.
Not long after the departure of the Syrians, who had left Joash, according to Second Chronicles twenty-four, verse twenty-five, with many wounds, his servants formed a conspiracy against him and upon his bed slew him in the house Millo, which goeth down to Silla. This description of the locality is perfectly obscure for us. The conspirators were Jozachar, the son of Shimeath, and Jehozabad, the son of Shomer, as in Second Kings twelve, verse twenty-one. However, according to the Chronicles in verse twenty-six, they were Zabad, the son of Shimeath the Ammonitess, and Jehozabad, the son of Shimrith the Moabitess.
Compare the marginal reference Joash, either from suspicion of intended treason or some other unknown cause, took up his abode in the fortress of Millo, as in First Kings nine, verse twenty-four. This conspiracy connects with religion. Soon after the death of Jehoiada, Joash apostatized. He had renewed the worship of Baal despite many prophetic warnings that had persisted in his evil courses, even commanding Zechariah to be slain when he rebuked them in Second Chronicles twenty-four, verses eighteen through twenty-seven.
The conspirators, who wished to avenge Zechariah, no doubt wished also to put down the Baal worship. In this, it appears that they succeeded. For, though Amaziah punished the actual murderers after a while in Second Kings fourteen, verse five, yet seems not to have been a Baal-worshipper. The only idolatries laid to his charge are the maintenance of the high places as in Second Kings fourteen, verse four, and a worship of the gods of Edom in Second Chronicles twenty-five, fourteen through twenty.