Capote didn't build suspense through the novel. It was only really like a fiction piece because of the extent of the character development, and the skill that Capote has at description. Otherwise, I felt like I was been told a story, rather than being shown a story, or rather than a story being unwound around me. I didn't like the long monologues, or the big slabs of criminal/police information.
This book is also about the loss of innocence that was occurring in many communities around the world at the same time. The middle of the 20th century seems to be when people gradually became less trusting, less willing to let their children roam freely, and more likely to lock their doors.
The killers, I believe, were both sane. I think they had personality disorders, but they certainly were aware of what they were doing - they just didn't care.
I don't think that this book makes the criminals heroic, or gives them any particular unwarranted attention. What this book does is stop people forgetting about one of many murders that occur every year. It reminds you of the wider affect that murder has on a community, or even a whole state.