Joash's son Amaziah takes the throne next. His first order of business is to kill the servants who killed his father. But don't kill their children; that's against the rules of Moses.
All in all, Amaziah is a good guy, but he just isn't really feeling the whole God thing. This gets him into trouble later.
King Amaziah gets an army together to attack Edom. He assembles men from Judah, but also pays soldiers from the Northern Kingdom of Israel to fight for him. He's feeling pretty good about this battle.
But a man of God comes to the king and lets him know that fighting alongside men from Israel is a bad idea. So the king listens and sends the soldiers from Israel back home.
These guys must have been itching for a fight, because they're not happy and go back home in a rage.
While Amaziah defeats the Edomites (with the help of God, of course), the dejected army from Israel attacks cities in Judah, killing people and stealing their stuff.
King Amaziah obviously never took Kings of Judah 101 because he goes on to make the same mistake that his ancestors made before him—he starts worshipping other gods. The gods of Edom to be precise.
Why, God wonders, would the king want to worship the gods of Edom? Didn't he just smash them to a bloody pulp in battle? Obviously, their gods are useless.
But King Amaziah just doesn't get it.
Later, Amaziah wants to try to make peace yet again with the Northern Kingdom of Israel. The king in the north pretty much snubs him, though.
Amaziah attacks Israel and ends up taking a beating (which he totally deserved for worshipping those other gods). The King of Israel tears down the wall of Jerusalem and carries all the Temple's gold and silver back to Samaria.
Years later, King Amaziah is killed in a conspiracy after he runs from Jerusalem.