Then all the people went to the house of Baal, and tore it down: Then correctly renders the common Hebrew conjunction as a temporal connector here. All the people includes Jehoiada and Joash as well as the other people of Judah. The house of Baal refers to the temple where the pagan god Baal was worshiped. This temple may have been in Jerusalem, perhaps near the LORD’s Temple itself or the royal palace. Or it may have been outside of Jerusalem in the nearby countryside. For the pagan god Baal, see the comments on 2 Chr 17.3.

His altars and his images they broke in pieces: The pronoun his refers to Baal. The first half of this verse may be rendered “Then all the people went to the place where they worshiped Baal and they destroyed it, demolishing the altars and idols.”

And they slew Mattan the priest of Baal before the altars: The archaic verb slew has been appropriately replaced by “killed” in New Revised Standard Version. Mattan is described as the priest of Baal. But the definite article may be misleading since there were many priests of Baal. Some interpreters think that Mattan may have been Baal’s chief priest.

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on 1-2 Chronicles, Volume 1. (UBS Helps for Translators). Miami: UBS, 2014. For this and other handbooks for translators see here .