Thus Joash the king did not remember the kindness that Jehoiada, Zechariah's father, had shown him, but killed his son. And when he was dying, he said, "May the LORD see and avenge!" At the end of the year the army of the Syrians came up against Joash. They came to Judah and Jerusalem and destroyed all the princes of the people from among the people and sent all their spoil to the king of Damascus. Though the army of the Syrians had come with few men, the LORD delivered into their hand a very great army, because Judah had forsaken the LORD, the God of their fathers. Thus they executed judgment on Joash. When they had departed from him, leaving him severely wounded, his servants conspired against him because of the blood of the son of Jehoiada the priest, and killed him on his bed. So he died, and they buried him in the city of David, but they did not bury him in the tombs of the kings. – 2 Chrn 24:22-25 (ESV)
Oh Joash! How far you have fallen! This is a grim reminder for us to persevere or we too could forget about following the Lord and turn towards dastardly deeds. This story is so sad – Jehoiada risked so much to save this boy and trained him up in the way of the Lord (2 Chrn 24:2). Now his own son is killed by him. The key for us is that God will avenge; the Lord will not let evil be unaccounted for. We must realize that it is not our place to take revenge for the wrongs done to us. But vengeance will come from the Lord, who loves justice and is powerful to save.