To be honest, I didn't finish the book, I only read about a third of it. I liked many things about the book, that it was one of the first in its genre or the writing style. But I didn't like the way it focused so much on the killers.
I know it's very important for crime investigation and prevention to understand why and how such crimes happen, but this book seemed to take an almost paternalistic approach. It seemed to be saying "these poor things, what else could they do but crime?" and it ended up sickening me, which is why I dropped it.