When you do something in “cold blood” you do it without being emotionally involved; in other words, you are detached. The murder is thought of in advance and the plans are executed in a ruthless manner, without the display of any emotion. You act like a cold-blooded animal.
The child was killed in cold blood in front of his house.
Does the temperature of the blood in our body increase and decrease depending on our mood? Not really, but people who lived some three hundred years ago believed that our frame of mind determined the temperature of the blood. The common belief was that when an individual became excited or angry, the blood within the body began to boil. When he remained calm, the blood became cool. These beliefs are reflected in everyday expressions like “hot-blooded” and “in cold blood”. I understand in the 17th Century, the expression “in hot blood” was used to refer to murders committed with passion. For some strange reason “in hot blood” went out of use. Scholars believe that the expression “in cold blood” is a translation of the French “sang-froid”. The expression “in cold blood” first appeared in print in 1711 in Joseph Addison’s “The Spectator”.