The king on the throne is a man by the name of Joash.
When king Joash was a baby, his grandmother (Athaliah, the daughter of Jezebel) murdered all of his brothers so as to wipe out the descendants of King David. She did succeed in killing them all - all but Joash. Little Joash was taken by his aunt to the temple, where he was hidden and raised until he turned seven. It was then that the priest Jehoiada, with the help of the other Levites and men of Judah, restored the throne to the young king.
Many years has passed since then, however.
Since the death of the priest Jehoiada, the state of affairs in the nation of Judah have been in decline. King Joash, the same one who had been rescued by the priesthood and kept safe in the temple, turned away from supporting the house of God. Instead, he was now swayed by idols and by other ideas. God had sent prophets to the king and the people, but the prophets were not heard out.
Now, the word of the LORD came upon the son of priest Jehoiada.
Jehoiada, the priest who had kept watch over and restored the throne to Joash, had a son of his own. And now this son, named Zechariah, was anointed as a prophet to warn the king.
Yet, the king did not listen.
King Joash did not listen even to this man who may have been like a brother to him. No, the king approved of orders to have Zechariah stoned to death in the courtyard of the temple.
While the Bible ends here, one commentary records how "Jewish tradition says that after Zechariah's death a fountain of blood appeared in the temple that not even the slaughter of thousands of priests could appease for the outcry of blood."
What a disturbing and powerful event...
You can read all the details here: 2 Chronicles 22:10-24:27
By the way, the icon depicts priest Jehoiada and I think Jehosheba (his wife) and the young Zechariah at the time King Joash took the throne.