And he did what was right in the eyes of the Lord, yet not with a whole heart.
2 Chronicles 25:2
This is an ominous life summary. Amaziah was so close, but because he was only half-hearted in his love of God, he ultimately suffered great loss. All of the victories his army gained on the field of battle were not quite spectacular. He was only half concerned about it and his army reflected the same self-concern, with Ephraimites looting Judah's own cites on the way back to Jerusalem (see 2 Chronicles 25:10-13).
His casual attitude toward God became an inlet for idolatry. Idols captured in battle captured his imagination and were worshiped by the king, resulting in God's anger on the nation (2 Chronicles 25:14-16). When God confronts the king with a prophet's word, Amaziah threatens to kill the prophet.
In the end half-heartedness is just idolatry with a mask. He is undone by an embarrassing defeat at the hands of Joash king of Israel that removed all power from Judah. After a few years as a puppet king, Amaziah is killed by a palace conspiracy (2 Chronicles 25:25-28). His story never has a good point to it. He did not give God all his heart.