Killing field a place where a heavy loss of life has occurred, typically as the result of massacre or genocide during a time of warfare or violent civil unrest. The term is particularly associated with Cambodia under the rule of Pol Pot and the Khmer Rouge in the late 1970s, where mass deportations from the towns to the countryside were followed by mass executions in what became known as the killing fields. It was with the release in 1984 of the film The Killing Fields that the phrase passed into the language.
killing no murder proverbial saying, mid 17th century; originally, the title of a pamphlet by Edward Sexby (d. 1658), which was an apology for tyrannicide and which was ironically dedicated to Oliver Cromwell. Captured and imprisoned, Sexby died in the Tower of London on 13 January 1658.
See also there are more ways of killing a cat.