You can really tell that this book was written before the term "psychopath" was well-known. Capote is clearly absolutely fascinated and horrified, and maybe confused, by the actions of these two serial killers and career criminals.
Personally, when it comes to true crime I prefer a book that gets right to the point and just tells us what happened and who did it and spends a little time exploring the why. Also, some homage to the victims' lives is greatly appreciated. I think Capote delivered everything I wanted, but in such a drawn-out fashion that frankly I got bored. I do not care what the weather was like unless it is directly relevant to the story. Every small town is shocked that this could happen to them. No family deserves to be murdered. Move on. Just tell me what happened.
However, taking into account that this book was written in the 1960s I can forgive Capote for being shocked. It was pretty new information for a lot of people back then that, hey, some people are extremely messed up and sometimes horrible things happen to really good people for absolutely no good reason.