Something done in cold blood is done without emotion. Murderers are often described as acting in cold blood.
Love and hate are both described in terms of heat, and a lack of emotion is considered cold. So if something is done in cold blood, it is done with no emotion whatsoever. Often, this phrase is used for actions that are somewhat inhuman. There are many descriptions such as "He gunned down the innocents in cold blood." This idiom is so popular that writer Truman Capote entitled his famous crime novel about an unexplained murder of a rural Kansas family, In Cold Blood: A True Account of a Multiple Murder and Its Consequences.