The definition of murder, according to most legal dictionaries, is «the unlawful killing of a human being with malice aforethought.» (redeeminggod.com)
Murder is the unlawful killing of another human without justification or valid excuse, especially the unlawful killing of another human being with malice (myintimcity.com)
It is true that a variety of widely accepted norms, including the keeping of certain promises, the abhorrence of unjustified killing of human beings, and perhaps even the sanctity of property rights, promote the adaption of the human species to its environment. (firstthings.com)