That's the thanks King Joash showed the loyal Jehoiada, the priest who had made him king. He murdered Jehoiada's son. Zechariah's last words were, "Look, God! Make them pay for this!"
A year or so later Aramean troops attacked Joash. They invaded Judah and Jerusalem, massacred the leaders, and shipped all their plunder back to the king in Damascus.
The Aramean army was quite small, but God used them to wipe out Joash's large army - their punishment for deserting God, the God of their ancestors. Arameans implemented God's judgment against Joash.
They left Joash badly wounded and his own servants finished him off - it was a palace conspiracy, avenging the murder of the son of Jehoiada the priest. They killed him in his bed. Afterward they buried him in the City of David, but he was not honored with a grave in the royal cemetery.
The temple conspirators were Zabad, whose mother was Shimeath from Ammon, and Jehozabad, whose mother was Shimrith from Moab.
The story of his sons, the many sermons preached to Joash, and the account of his repairs on The Temple of God can be found contained in the commentary on the royal history. Amaziah, Joash's son, was the next king.