Amaziah in Fausset's Bible Dictionary

1. Son of Joash; on his accession to the Jewish throne punished his father's murderers, but not their children (Deuteronomy 24:16); a merciful trait of character, which it is implied other kings had not. He had reigned jointly with his father at least one year before Joash's death; for 2 Kings 13:10 compared with 2 Kings 14:1 proves he reigned in the 39th year of Jonah of Judah; 2 Chronicles 24:1 shows that Joash of Judah reigned 40 years; therefore Amaziah must have been reigning one year before Joash's death, The reason comes out in that incidental way which precludes the idea of forgery, and confirms the truth of the history. In 2 Chronicles 24:23; 2 Chronicles 24:25 we read: "the host of Syria came up against him (Joash) ... to Judah and Jerusalem, and destroyed all the princes; ... and when they were departed (for they left him in great diseases) his own servants conspired against him for the blood of the sons of Jehoiada the priest, and slew him on his bed." The "great diseases" under which Joash labored, at the time of the Syrian invasion, were no doubt the cause of Amaziah his son being admitted to a share in the government. Blunt well observes how circuitously we arrive at the conclusion, not by the book of Kings alone nor Chronicles alone; either might be read alone without suspicion of such a latent congruity. He slew of Edom in the Valley of Salt (S. of the Dead Sea, the scene of David's general's victory: 2 Samuel 8:13; Psalm 60 title; 1 Kings 11:15-16; 1 Chronicles 18:12) 1 Chronicles 18:10; 1 Chronicles 18:000, and his forces threw 10,000 captives from the rocks, and he took Selah or Petra their capital, which he named Jokteel (the reward of God) after a Jewish city (Joshua 15:38). Then he showed that, whereas he partly did "right in the sight of the Lord," it was "not like David his father, with a perfect heart" (2 Chronicles 25:2; 2 Kings 14:3)...

Link: https://bible-history.com/faussets/A/Amaziah/