It turns out that it's actually the policeman's brother (and patient at the asylum) who's involved in the murder (and a number of other particularly gross murders that occur in the book). The NYPD naturally isn't that bothered if they catch dead or alive. Carson Ryder, the cop, isn't so keen on the dead idea, so tries to catch his brother before the police do. The Alabaman then sets off to do just that.
On the face of it, the book is daft. Who's ever heard of a genius mass murderer being related to a bright cop for example? But that having been said, the pace of the story helps you forget its' basic daftness.
If you can accept the basic silliness of the story, you'll enjoy this book.