II Chronicles - Chapter 25
Amotz and Amatzyahu
Amatzyahu, who succeeded Yoash, was a righteous king. When he was securely established in his role, he executed those who had killed his father, but he did not avenge himself on their families. (Apparently, he would have been able to do so in the case of a rebel, but he declined to exercise this option, possibly because his father kind of deserved it.)
Amatzyahu organized his people into military units, prepared for war. He also hired soldiers from the kingdom of the Ten Tribes to bolster his forces. A prophet warned them not to go to war with the troops of the northern kingdom, since G-d was not with them. (The Radak cites a tradition that the prophet was Amatzyahu's own brother, Amotz, the father of Isaiah.) Amatzyahu balked because he had already paid the hired forces. The prophet assured the king that G-d could more than make up his investment. So Amatzyahu sent the army of the north home, which made them very resentful.
Amatzyahu led his forces to war against the army of Seir. They captured 10,000 of the enemy, whom they tossed off a cliff. (This did not please G-d, Who did not sanction such a brutal method of execution for prisoners of war.) Meanwhile, the disgruntled troops that Amatzyahu had dismissed sacked Judah while he was busy elsewhere. They killed 3,000 people and looted the cities.
When Amatzyahu returned from his campaign, he took the idols he had captured and set them up for his own use. G-d REALLY didn't like this, so he sent a prophet to ask Amatzyahu just what, exactly, he was thinking. The king, thinking that the prophet was speaking on his own behalf, told him to shut up. The prophet replied that G-d had sent him and He would destroy Amatzyahu.
After mulling things over, Amatzyahu sent a message to King Yoash of the Ten Tribes challenging them to battle. Yoash laughed at the prospect and replied that the army of Judah would be more than outclassed by his forces. Amatzyahu would not relent, so Yoash met him on the battlefield. Sure enough, the army of Judah was beaten down. Yoash captured Amatzyahu and brought him back to Jerusalem, where he proceeded to plunder the palace and the Temple.
Since Amatzyahu had turned to idolatry, some of his subjects rebelled against him, forcing him to flee to Lachish. They eventually caught up with him and executed him. He was returned to Jerusalem for burial.