In this chapter, Hazael becomes king over Aram and Ahaziah becomes king over Judah.
This chapter is a bit diverse; it covers several different stories and I don't think it has just one theme, but if I had to choose just one, it would be "royal transitions" as Aram gets a new king and Judah goes through two kings.
The first story is only barely related to the rest of the chapter. We see the unnamed woman from 2 Kings 4:8-37, whom Elisha first gave a son and then raised that same son from the dead. Like what I discussed in 2 Kings 4, we can see that the woman maintained her friendship with the prophet and in this case she benefits from it tremendously when he warns her about a coming famine. It is likely that the story here is retrospective and that the seven years of famine are the same as the famine that has been striking Israel for some time now. For instance, 2 Kings 4:38 mentions a famine in the land, and this could be the same famine being discussed now.
What happened is that while the woman was out of the land, some person or family moved into the woman's home and took over her fields and now she is petitioning the king to get it back.
Interestingly, Gehazi appears to still be the servant of Elisha. It's possible that this part of the story is also retrospective (i.e. it occurred before Gehazi was cursed in 2 Kings 5:27), or it's possible that Gehazi remained Elisha's servant even after he was struck with leprosy. I haven't read any definitive commentary that leans one way or another. Personally, I think it's unlikely Gehazi would have remained Elisha's servant after being struck with leprosy because the laws of ceremonial cleanliness would render Elisha unclean every time he was with Gehazi. This would be very disruptive to Elisha's ministry. Of course, it's even harder on Gehazi, but he received leprosy as punishment for misusing Elisha's authority as prophet and his own authority as Elisha's servant, which is yet another reason why Elisha should remove Gehazi as servant, because he proved himself untrustworthy.
I think the more interesting part of this story is how it shows the relationship between the king and Elisha. One of the commentaries I read suggested that the "king" mentioned in this story could be Jehu on the assumption that Joram (the previous king of Israel) would have been familiar with Elisha. Either way, I would personally interpret this as a sort of rapprochement between the king and Elisha. The king is expressing interest in Elisha's acts, and when he learns about what Elisha did for the woman he grants her a favor. Since we have already learned the woman has a close relationship with Elisha, the king granting her a favor shows that the king is at least somewhat amenable to Elisha's activity.
After some consideration, I guess this is something we have always known; in the past, the king has always been pleased whenever Elisha did something in his favor (like deliver him from the Arameans) and always responded with hostility whenever things went poorly (like when they were besieged by the Arameans). The king is taking more interest in Elisha, but I would not be surprised to see the king turn on him when things deteriorate again.
After this, Elisha goes to Damascus in order to anoint Hazael as king of Aram (which Elijah had been commanded to do back in 1 Kings 19). This is the fulfillment of at least two commands from God, the first that Hazael would reign over Aram and the second (from 1 Kings 20:42) is that Ben-hadad should die.
There are parallels between this story and the story from 2 Kings 1 when Ahaziah sent messengers to query the prophets of Baal if he would live. In both cases, there is a sick king asking after his health. Ahaziah sent a messenger to ask the prophets of Baal, while Ben-hadad sends a messenger to ask the prophet of the LORD. Neither king turned to his own god. In both cases, the king dies.
This passage is interesting to me not because Elisha tells Hazael to lie to Ben-hadad, nor because Hazael goes on to murder Ben-hadad (which if you read carefully, Elisha did not tell him to do), but because Elisha was going to anoint Hazael even though he knew that Hazael would harshly treat the Israelites, his own people. As I just mentioned, God commanded Elijah to anoint Hazael back in 1 Kings 19:15, and in 1 Kings 19:17 it is strongly implied that Hazael would be a force of judgment upon Israel, putting them to death with a sword. This language is perhaps figurative, but nonetheless threatening.
It is more likely than not a judgment upon Israel's idolatry, but Elisha is genuinely moved with compassion for his own people, weeping openly before Hazael with the prophetic foreknowledge of what Hazael would do. Hazael, for his part, proves himself to be something of an anti-David. After being anointed king, Hazael takes the matter into his own hands and murders his master, the former king Ben-hadad. David, on the other hand, continued to serve Saul even after Saul tried to murder him on several occasions. In this way, Hazael has already shown that he is not a righteous king like David, nor will Hazael follow the LORD. The LORD is anointing Hazael as king not because of his moral character, but because Hazael will help fulfill the LORD's judgment upon Israel. Would it have been better if Hazael were a righteous king? Sure. But God can't change that, at least not right now. In large part that is a matter for Hazael to work out in his own life. But just because Hazael is not a righteous person does not mean that God can't use him. God has shown over and over that he can use wicked men to fulfill his purposes and gradually build towards the redemption of mankind.
Think of Pharaoh. In Exodus 4:21 (and elsewhere) God says that he would harden Pharaoh's heart so that God could perform great miracles and work a great deliverance for Israel with Pharaoh standing in opposition. It would have been better for Pharaoh and all of Egypt if Pharaoh had been a righteous man and had allowed Israel to depart in peace. It would have been better if Pharaoh had not oppressed Israel and enslaved them. Even though God can "harden his heart", God cannot change Pharaoh's fundamental character or desires, which means that the only question is how God will respond to Pharaoh's wickedness. What God shows is that his salvation, his purposes and his plan for human redemption cannot be derailed by human wickedness, whether Pharaoh's or Hazael's. Pharaoh stood against the plans of God and God simply co-opted him into his plan of salvation. Hazael is not quite the anti-hero like Pharaoh, but he is a murderous and self-serving man and if God isn't going to save him, he can still use him in the bigger plan.
In verse 18 of this chapter, we can see the continued political integration between the northern kingdom Israel and the southern kingdom Judah when the new king of Judah marries a daughter of the king of Israel. This political integration began with the alliance between Jehoshaphat and Ahab and it is secured in the next generation by marriage. Verses 18 and 27 tell us about the poisonous effect of this alliance as the house of Ahab leads the kings of Judah deeply into idolatry, contrary to the ways of the LORD.
Verses 19-20 tell us that God refused to destroy Judah for the sake of David, but that he would punish and weaken Judah by inciting revolts in their subservient neighbors. Note that even though verse 22 tells us that Edom revolted "until this day", the day in question is when this passage was written. It's possible that Edom could later be re-conquered by Judah after this passage was authored.
Furthermore, in verses 28-29 we see Judah and Israel fighting together against Aram and the king of Israel is wounded in battle. Ahaziah, a friend of Joram, goes to visit him in Israel, and this sets the stage for the next act of divine judgment upon the idolatrous rulers of these two houses.