Originally, 2 Kings and 1 Kings were part of the same book in Hebrew. But later on, after the advent of Christianity, they were split in half. So that explains the "2" part of the title. Really, though, the book is called "Kings" because… uh, it's about kings. There are a lot of kings in this book—and they're usually pretty bad.
In fact, the book takes a somewhat black-and-white view of these kings. If you worship God alone, without any images or high places or golden calves or anything, you're a good king. But if you worship in any of these ways or add in foreign gods, you're a bad king. From a Biblical perspective, this is all fair enough. Part of the point of the book is to try to determine where it all went wrong, to discover which kings led Judah and Israel well and which kings led them astray.