The ending of 2 Kings is a major disaster for Judah and Israel. First, the Assyrian King Shalmaneser devastates Israel and sends everyone into exile, before repopulating it with people sent in from different countries (like Babylon and Cuth). Then, at the very end, we get the major disaster of Jewish history (up to this time, before the Romans destroy Jerusalem, and before the Holocaust). The Israelites from Judah are sent into captivity in Babylon—a subject covered at length in the Book of Daniel.
This is a huge catastrophe. Basically, God lets everything get destroyed: the Temple Solomon built for him, the legacy of David, and more. Only the faithful people who endure the exile are able to keep the fundamentals of the religion alive. The rest of 2 Kings, leading up to this, is really an attempt to explain this catastrophe, to try to show how the misbehavior of the kings and their people led them to this state.