There are bad betrayals and good betrayals in 2 Kings. The bad betrayals involve the various conspiracies that bring down different kings. For example, when Hazael smothers the sick king Ben-Hadad of Aram with a wet piece of cloth, we can safely say that that's, well, pretty low. But God commands Jehu to essentially betray Joram and kill Jezebel and Ahab's whole family. It would normally be an act of treason, but instead God makes it good, sanctioning it. The same is true when Jehoiada orders Athaliah's death. She got into power by slaughtering her own family members (not good), hence committing treason against her is really the right thing to do.
While betrayal is clearly a really bad thing (duh), its opposite quality—loyalty—is usually rewarded and praised by God (yay). But the people of 2 Kings need to go through trials of faith and difficulty to remain loyal—the Shunnamite woman whom Elisha helps would be a good example. She's clearly one of the most courageous characters in the book. Also, different kinds of loyalty are demanded from different people. Naaman can still bow down in the temple in Aram when he needs to, for example. The expectations for the way kings and prophets show their loyalty are different.
In the song "Power," Kanye West poses the question "You got the power to let power go?" That question basically means the same thing as, "Do you have the power to remain humble?" The people who do have that power tend to do pretty well in 2 Kings. Elisha wields huge amounts of power—he can even resurrect the dead, but he's extraordinarily humble. His entire life revolves around God.
At the same time, there are powerful people who don't have the power to surrender in some way. Sennacherib and Manasseh are good examples of this tendency: their bread-and-butter is murder and destruction and pride. But the fate of Sennacherib demonstrates where this leads: he boasts about defeating God, and then is utterly crushed. His own sons end up murdering him.
War: all the time. There's tons of warfare in 2 Kings. War is, like, the main thing that kings do—it's all over the place. Sometimes these wars are fought for selfish or trivial or stupid reasons—like when Amaziah fights Jehoash of Israel out of pure brashness. But at other times, wars are fought for self-defense or to advocate for some higher moral cause. God himself is a warrior at times, fighting to assert his own invincible will over the smaller, individual wills of mortals.
Considering that this is, you know, the Bible, religion is a pretty big theme. In this case, it has to do with the clash between different interpretations of ancient Israelite religion. The people following the "sins of Jeroboam" in Israel, for example, probably thought that they were worshipping God in the correct manner. Some scholars think that the golden calves created by Jeroboam I weren't supposed to actually be gods, but were supposed to represent them. And the foreigners who move in to Israel from other countries end up worshipping Israel's God along with their own.
But 2 Kings advocates the Deuteronomy-inspired picture of faith: God is one, and cannot be worshipped through depictions or images or sacred poles. Implicitly, God can only be fully worshipped at the Temple in Jerusalem. That's why the high places and Jeroboam's golden calves are considered to be so bad. 2 Kings is a great place to study the way these sort of theological wrestling matches take place, since it often ends up involving real physical conflict.
Sin, for the authors of 2 Kings, is a form of forgetfulness, you could say. When people forget God, they tend to fall into sin. This takes the form of engaging in various prohibited religious practices, but also in generally selfish deeds. Gehazi tries to take advantage of a situation for his own benefit by fleecing Naaman and ends up being cursed with leprosy, for instance. People who commit the worst sins tend to be putting themselves at the center of their world—taking a giant swim in Lake Me—instead of some higher cause or reality.
Justice and judgment provide the response to the previous category: sin. Justice is what punishes or cleanses sin, after all. The God of 2 Kings is big on justice, though he's not without his merciful side, as well. Sometimes, though, the punishment might not seem to fit the crime (at least, to the contemporary reader), like when a crowd of children mocks Elisha and are then attacked and killed by wild bears. (Overreaction much?)
Justice, in the Biblical sense, frequently takes on some pretty hair-raising forms, like when Josiah slaughters disobedient priests on their own altars. The ultimate act of justice in 2 Kings may be the most devastating: the departure of Judah into exile. The mercy of God tends to resolve things in a relatively peaceful way, like when he extends the life of Hezekiah. But justice is harrowing. There may be a greater purpose lying behind such retribution, leading towards a time of peace (as Isaiah and other prophets predict). Yet, as 2 Kings ends, it seems like that time is still very far off...