“When the Arameans had withdrawn, leaving King Joash severely wounded, his servants conspired against him because of the blood of the son of the priest Jehoiada. They killed him on his bed so that he died. They buried him in the city of David, but they did not bury him in the tombs of the kings. Those who conspired against him were Zabad son of Shimeath the Ammonite, and Jehozabad son of Shimrith the Moabite. Accounts of his sons, and of the many oracles against him, as well as the rebuilding of the house of God are written in the Commentary on the Book of the Kings. His son Amaziah succeeded him.”
This section explains in detail what is mentioned in 1 Kings, chapter 12, about the tragic sudden death of King Joash. There it simply says that there was a revolt by his servants that killed him. There, without giving any details, his servants rose up against him. Led by Jozacar and Jehozabad, they killed him. There King Joash was killed at the Millo or fortress that goes to Silla, while here he is killed in his bed. Here there is a simple explanation. This act was to revenge the death of Zechariah, the son of Jehoiada, the old high priest. Here the names of those who killed him were Zabad and Jehozabad, as Jozacar has become Zabad. Notice that they were Ammonites and Moabites, not people from Judah. However, they buried him in Jerusalem, but not in the tomb of the kings where Jehoiada was buried. His son King Amaziah took over after him, so this was not a dynasty revolution, put a personal one. If you want more information about King Joash, the lost “Commentary on the Book of Kings” would be helpful. Here is the first mention of the biblical Book of Kings that this biblical writer was revising. It no longer is the lost book about the annals of the kings of Judah.