'In cold blood' means to do or execute a cruel act without any feeling of compassion. It refers to committing a heinous or gory deed without any semblance of guilt or mercy, utterly ruthless. For example, "he was killed in cold blood by the Mafia". An act done in cold blood refers to an act that is callous and cold-hearted. A cold blooded person is usually cold and unemotional. He is generally not affected by emotion, neither does he show any.
This is actually a metaphorical usage of the word cold-blooded derived from cold-blooded animals, meaning a person who basically does not possess warmth.
The term Cold Blood is used to describe an account of a gruesome crime, principally in case of a brutal killing or a murder. Cold blood is a word usually picked to describe a horrific execution of innocent victim, so drastically unsettling that it makes it doubtful to imagine the perpetrator to be a human being.
Cold Blood is also origin of the term 'Cold Blooded' addressed to animals like reptiles, fish or other amphibians and spiders, which were said to have cold blood due to a belief that these animals were incapable of maintaining their body temperatures as per varying environment. However, with time usage of this term has lessened and more so, after it was discovered that these animals certainly have an ability to adapt their body temperatures to match their habitat by borrowing under soil to avoid heat, using sunlight, using gills and fins to absorb and feel heat or changes in surroundings and then match the body temperature accordingly.
It means lizards are evil