In this chapter, Ahaziah becomes king and reveals himself to be just as much of a catastrophe as his father Jehoram.
Similar to the previous chapter, the devastation in this chapter can be directly traced back to king Jehoshaphat, who had deliberately cultivated a political alliance with Israel by intermarriage. While king Jehoshaphat suffered both military and economic loss for his decision, it is not until the lives of his children and grandchildren that the full gravity of that mistake is made evident. The most important element of Jehoshaphat's mistake is that he imported Israel's culture of betrayal and intrigue into Judah's royal court. In the previous chapter, this resulted in the murder of Judah's royal princes by the new king Jehoram, and in this chapter it results in the murder of many others.
To begin with, the new king Ahaziah maintains Jehoshaphat's policy of alignment with Israel, almost certainly due to the influence of his Israelite mother Athaliah (v. 3-4). Similar to Jehoshaphat, Ahaziah also goes to fight a battle alongside Israel against Aram (v. 5), and similar to Jehoshaphat the battle goes poorly, with Israel's king Jehoram wounded in the fight. We can presume that the battle was another victory for Aram. Note that the Jehoram in this chapter is a different person with the same name as the previous king Jehoram of Judah. Neither Jehoram nor Ahaziah die in the battle directly, but it brings them both to the same place and time when Jehu marches to enact God's judgment upon the house of Ahab. Although it might seem like Ahaziah is an innocent casualty in Jehu's crusade to wipe out Ahab's descendants, that's actually not quite true. Ahaziah himself is a grandson of Ahab through his mother Athaliah, and therefore Ahaziah is subject to the same condemnation that befalls Jehoram. The full details of this story are given in 2 Kings 9, as the rendition in this chapter is somewhat abbreviated.
From a certain perspective, this is just compounding difficulties for Judah. It was already bad before when Jehoram (Jehoshaphat's son) killed all of his brothers or when Judah suffered not one but two defeats at the hands of the Arameans while allied with Israel. In addition to all those things, now Ahaziah is under God's judgment and he is put to death by Jehu.
We might think that things could hardly get worse for Judah, but I actually think there is a silver lining in this chapter. As difficult as it may be, the death of Ahaziah is actually part of the redemption process for Judah. If Ahaziah remained king, he would have continued following exactly the same policies that brought Judah to this situation in the first place, and Judah would have never escaped the domination of their northern neighbor, to their own detriment. On the other hand, if Ahaziah and Athaliah are wiped out and a new generation of leaders are raised up in Judah, it is possible that they will be able to "reboot" the stable and comparatively God-centered royal culture that Judah possessed before Jehoshaphat.
After the death of Ahaziah, you might think that the nightmare is over, but unfortunately it is not. Athaliah is still alive, and in a bizarre twist, she sees the death of her son as an opportunity to seize power for herself. In verse 10, rather than let power fall to one of Ahaziah's sons, she kills every royal prince she can find and takes the throne for herself. This is horrible, and at least a little crazy, but perhaps not entirely surprising. Athaliah had already directed her husband, the previous king Jehoram of Judah, to kill off a broad swath of Judah's royal family for very similar reasons. Given an opportunity to take power, Athaliah simply does that same thing again, killing every possible challenger and ascending to the throne.
For a while, it seemed like Athaliah might be successful. She reigned for six years and was relatively unchallenged. Perhaps through Athaliah, the house of Ahab might survive in Judah and God's judgment on Ahab might be deflected. There's just one thing that went wrong in Athaliah's plans, one small, young detail, perhaps not even a year old. That detail's name is Joash, one of the last surviving remnants of David's house, and in the next chapter the LORD will overthrow Athaliah through Joash and the high priest Jehoiada.