I have suggested elsewhere that one important theme in Truman Capote's book is the way two young men may be relatively harmless when they are separate but can make a lethal combination when they get together. They are like two chemicals that are innocuous in separate bottles but can create an explosion when mixed together. Dick Hickock and Perry Smith are small-time delinquents who are both trying to impress each other with how BAD they can be. Neither one of them would have committed such a horrendous crime by himself, but when they get together they goad each other into doing terrible things to innocent people. The fact that it took two of them acting in tandem to commit the murders also made it possible for the police to catch them and get them to confess. A similar thing happened in the so-called Crime of the Century in the 1920s, when Loeb and Leopold murdered a young boy just for the "thrill." Leopold later wrote an interesting book about it titled 99 Years to Life. Capote was undoubtedly familiar with it.
First of all, of course there is a theme. All works of literature have them. Theme can be defined as "a general statement of universal truth." When reading In Cold Blood however, you should be struck by the fact that there isn't just one truth. In this book, Capote is urging us to recognize that life is complicated, as is his novel, and cannot often be summed up with pat maxims.
In Cold Blood contains many themes, and one of them is this "nature vs. nurture" idea. Capote is a Post-Modernist writer who is building on the ideas of some Modernists who looked at character (personality) as something that can be created by several things: heredity, social conditions and environment. Capote builds on the ideas of earlier writers, who created fictional situations in which this could be true, by showing the reader that it may be true in real (non-fiction) situations as well.
Another theme in In Cold Blood must surely be the complicated nature of compassion. Throughout the book we are asked to have compassion and sympathy for so many of the characters--the Clutters, the town that will never again be as innocent as it was on the day before the murders, Al Dewey, Dick Hickock, Perry Smith, the Hickock and Smith families--indeed, nearly every character in the book deserves the reader's compassion. Yet this is complicated, since many of the people we feel compassion for are murderers or people whose actions clearly caused their own downfalls.
One of the main themes in Capote's book is the issue of nature vs. nurture. For example, did the killers become this way because of the environment in which they were raised, or is there some innate evil about these men that propelled their henious crimes? Through his extensive interviews, Capote tries to get to the bottom of this question. In the end, it seems that nurture may have played a larger role, but nature may also have played a significant part. Many people who have similiar background (alcoholic parents, suicide, abuse) do not go on to become killers. In Capote's estimation, I would argue that he sees Smith and his partner, Willy Jay, as being a lethal (literally) combination of the two: perhaps two-thirds nurture and one part nature.
Adding on to previous answers on the Nurture/nature sides~
Personalities, like looking at Nancy, she was just baking pies and days later her friend found her, covered in blood, shot.
Capote also left out the murder scene. The slaying and brutality would've been graphic, so having a sense of sympathy contributes to the nurturing side.
Look at the town, so quiet and innocent, to where people didn't need to lock their doors.
I took this as the realism, the non sugar-coated text.
Looking at the men who commited the slayings.
Traumatic upbringings. Cruelness. Their tattoos.
Things that made them seem rough; not only rough but real.
I guaruntee that half of my AP lang class will see this, probably googling the themes of In Cold Blood.