What is the origin of the idiom "in cold blood" and when to use it?

The idiom "in cold blood" originated in the late 19th century and gained widespread recognition through Truman Capote's true-crime novel of the same name, published in 1966. This book told the true story of a brutal murder in Kansas in 1959, where two strangers killed a family for no clear reason. The phrase "in cold blood" perfectly described how these killers acted – they were cold-hearted and showed no emotion or remorse. Over time, the idiom has become a powerful way to describe acts that are carried out deliberately, ruthlessly, and without compassion, whether in the context of crime or other actions lacking empathy.