In this chapter, Jehoram becomes king, does horrible things, and dies.
This is a difficult chapter to read, but given the action in the previous chapter, I think it's all too predictable. The previous chapter showed king Jehoshaphat reaching out to Israel a second time, even after the first disastrous battle that he fought on behalf of Ahab (2 Chron 18) and the prophet rebuking him for it (2 Chron 19). In spite of the LORD's urging, Jehoshaphat simply was not able to step away from his alliance with the house of Ahab (2 Chron 20:35-37). While Jehoshaphat suffers loss for these decisions, a much greater tragedy is about to unfold during the lifetime of his children.
Verse 6 makes it clear that while Jehoram was a king of Judah, he walked in the ways of Israel because he was married to an Israelite princess. One thing that's quite remarkable about Jehoshaphat is that throughout his alliance with Israel, he always maintained his personal integrity and devotion to the LORD. Jehoshaphat was making politically expedient decisions to improve his own kingdom's position, but was doing so in ways that are contrary to God's will. Even though Jehoshaphat himself maintained his devotion to the LORD, he was raising up his own son and the next generation of Judah's leaders in an environment where they could be shaped by the idolatry and treachery that has defined Israel's court politics. Regardless of what temporary political advantage Jehoshaphat gained, it is about to be profoundly broken by the devastation in this next generation, beginning right here with Jehoram.
This chapter begins in verses 1-7 when Jehoram becomes king, and in order to prevent any subversion to his reign, he kills all of his brothers and some of the other prominent leaders in Judah. This is an action that is totally nonsensical from the history of Judah; there have not been any rebellions in Judah since the days of Rehoboam. However, if we peek at the history of Israel that is represented in 1 Kings 15:25 through 1 Kings 16:22, we can see that no less than three kings and one contender for the throne were murdered within 26 years. Not only were the kings themselves killed, but oftentimes the entire royal family would be murdered as well. All of these assassinations occurred in the time just before Ahab and it's doubtless that the entire court of Israel was trained to expect usurpers to rise up, and even to usurp the throne themselves if an opportunity is present.
When verse 6 says that Jehoram "walked in the way of the kings of Israel", this actually tells us quite a lot about the action that is to shortly follow: Jehoram probably expected that his brothers might try to move on the throne, since that was a regular occurrence in Israel, and he sought to undercut any possible contenders by simply murdering anyone who could potentially challenge him. This is a mindset and an action that would only make sense for a king of Israel, and so it is clear that the culture of Israel's royalty has been transmitted to the new king of Judah.
Verses 8-20 are basically a long sequence of disasters that the LORD inflicts on Judah for Jehoram's sins, as well as the idolatry that Jehoram cultivates amongst the people. It is only interrupted in verse 12 when Elijah makes his sole appearance in 2 Chronicles to threaten death and destruction upon Jehoram. This warning is sandwiched between two passages describing hostile nations rebelling against Judah or invading her. Jehoram himself is struck with a painful, incurable disease and he dies from it. All of this is consistent with the LORD's promise in 2 Chron 7 that he would punish Judah if they ever sin against him, but heal them if they repent. There is no indication that Jehoram ever repented.
We might think that the worst is past when Jehoram dies, but unfortunately that is not the case. In the next chapter, Ahaziah becomes king in his father's place and Judah's collective nightmare continues.