To do something "in cold blood" means the opposite of having done that thing in the heat of the moment. A murder committed in cold blood is one that has been thought about, planned, and then executed without the influence of any kind of emotion. While a murder committed in the heat of the moment might be, to a certain extent, understandable because of the circumstances (if passions were running high), a cold-blooded murder is generally understood to be far more severe, not least because of what it says about the perpetrator.

The murder in this book was definitely committed in cold blood, and accordingly, the murderers were charged with and convicted of murder within a very short space of time. The murderers were all former criminals who knew each other from prison, connected with each other, and planned the murder as a means of stealing a large amount of money, with which they planned to start new lives. Although they observed that the victims' family were "sweet," this did not prevent them from committing the murder—one murderer, Smith, even thought the victim, Herb Clutter, was "nice" right up until he killed him.

The jury, too, were quick to convict and were unmoved by sentiment. Convicting in forty minutes, they were described as the opposite of "chicken-hearted"—so we can argue that their blood, too, was cold as they sentenced the murderers, knowing that they would be sending them to their deaths.

The title of Truman Capote’s 1966 true crime classic “In Cold Blood” works on multiple levels. Dick and Perry murdered the Clutter family for no real reason; they killed in a purposely ruthless manner, without feeling and with cruel intent, which is the basic definition of doing something in cold blood. However, Capote wants the reader to take the title one step further. Dick and Perry are executed at the end of the novel, and there are many people in America who think that execution is a form of murder. Perhaps Capote is suggesting that the American justice system killed Dick and Perry in cold blood. And finally, there’s Holcomb, Kansas. Before the Clutter murders it was a quiet small town with traditional values and a Norman Rockwell-like innocence. The Clutter murders and the trial and execution of Dick and Perry murdered a small town’s innocence in cold blood.