The priest of Baal and Queen Athaliah

The high priest Jehoiada arranged an agreement between God, the king, and the people. The king and the people would be God's people.

Jehoiada made a covenant between the LORD and the king and the people, that they should be the LORD's people; between the king also and the people. 2 Kings 11:17

And as God's people, they would destroy the religious symbols of all other Gods, and kill their priests. So God's people went into a murderous rampage. They started by killing Mattan, the priest of Baal, at the altar.

All the people of the land went into the house of Baal, and brake it down; his altars and his images brake they in pieces thoroughly, and slew Mattan the priest of Baal before the altars. 11:18

And finished by killing Queen Athaliah.

And all the people of the land rejoiced, and the city was in quiet: and they slew Athaliah with the sword beside the king's house. 11:20

The Friendly Atheist - How many people did God kill in the Bible?

The priest of Baal and Queen Athaliah