Lullaby tells the story of a killer nanny. It's not your typical thriller; the who, what, where, and how are all laid bare in the first (brief but intense) chapter. The rest of the book is about the why.
I think, despite the fact that it's deeply horrifying, the story is also very satisfying. Not, by any means, due to justification or retribution or resolution. It's because we're given the explanation. Think about it—any time something like this happens, what do people want to know? Why. How could she? And this book delves into the gritty details, the complex blend of circumstances that could lead to needless tragedy.
And explaining why fills an entire book. It's an extensive study of the main characters and their various relationships (but nowhere near as boring as that makes it sound), and not something that could be distilled into a headline. But really, as satisfying as it is to know the back story . . . knowing why never *really* explains it. Knowing why doesn't help as much as you'd think it would. I mean, it's not as if knowing why makes it acceptable. I suspect the same would be true for all of those horrifying crimes you hear about on the news.