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.