In this chapter, Amaziah becomes king, defeats the Edomites but then falls into idolatry.
The life of Amaziah, like so many of the earlier kings, is a story of two battles. The first battle goes reasonably well, but the second battle ends in disaster. Starting in verse 6, Amaziah takes a small but significant step towards re-aligning Judah with Israel again, like Jehoshaphat did back in 2 Chronicles 18. Certainly Amaziah is taking a much smaller step, only hiring troops from Israel rather than intermarrying with Israel's royal family, but it is a step nonetheless, and as the man of God makes clear in verse 7, it carries all of the same peril as Jehoshaphat's earlier decision.
In this case, even though Amaziah had already paid the mercenaries a fee upfront, he does the right thing and sends them away. The Israelites are enraged because even though they got paid and get to walk away with that money for free, part of their compensation would have been profit sharing from plundering the Edomites. Since they were rejected from the battle, they were not entitled to any of the plunder. The Israelites retaliate by raiding and pillaging all the towns of Judah that they encounter on their march home, basically trying to get plunder some other way since they were not permitted to invade Edom alongside Judah. Judah is harmed somewhat, but recovers.
Perhaps more importantly, because the Israelites attacked Judah, it may have precipitated the later battle between Judah and Israel in v. 17. Amaziah may have desired to fight Israel as revenge for the Israelite mercenaries who raided towns in Judah and killed 3,000 people.
Before that happens, there is an important interlude, which is the Edomite idols. This episode in v. 14-16 marks the turning point in Amaziah's life when things go from reasonably good to much, much worse. I also see this as part of Judah's gradual decline. Even though they made mistakes too and sometimes did evil things, I personally feel like the earlier kings (like Abijah or Asa) did not have as many problems as Amaziah, and Amaziah has fewer problems than a lot of the kings who come after him. Just as Amaziah suffers decline in his own lifetime, I think we see a progressive decline in Judah's society as a whole.
After this, Amaziah fights his second major battle against Israel. Like I said before, it's possible he wanted to fight Israel as revenge for their earlier raids. Regardless of the reason why, this battle has two practical effects. First, it cuts off any possibility of future alliances between Judah and Israel. Initiated by Jehoshaphat, the bilateral relationship between Judah and Israel really never comes back from this, which ironically is a good thing for Judah since it prevents future kings from falling under God's judgment of Israel again.
Second, this failed battle pretty much ends Amaziah's kingship. Even though he isn't overthrown immediately, it's obvious that the military defeat undermines Amaziah's political support and contributes to his eventual assassination in v. 27.
So what kind of conclusions can we draw about Amaziah's life as a whole? While Amaziah makes a handful of good decisions, overall it seems to me that Amaziah is a middle-of-the-pack kind of guy. He does some things well, does some other things poorly, and overall just seems to go about his affairs in whatever way he likes, with no regard for God, which in this case ends in total disaster, as it often does. We can't really condemn him for being a truly evil man, but he makes bad decisions and sticks with it, and this is usually how that kind of thing ends.
In the end, Amaziah dies and in the next chapter, his son Uzziah becomes king.