Murder is classified by humans as a subset of killing according to their cultural attitudes. In nature killing is an intrinsic and essential element of its self-recycling process, but for humans, killing is enmeshed with a complex and socially biased range of evaluations. The classifications of types of killing such as harvesting, slaughtering, euthanasia, revenge, murder, manslaughter, and so on, are generally quite different from one culture to another. Opposing intercultural attitudes confound even further the ways in which assassinations, war casualties, executions, and ethnic purges are to be evaluated.

Murder is usually the culturally internal classification of a killing which is legally disapproved of by the controlling authority and generally carrying a severe retribution. A culture never classifies the killing of battle enemies or espionage agents as murder. Murder is an in-house happening. The controlling authority of a society usually classifies a killing as that of murder when the act was deemed to be performed as a means of facilitating one or more individuals self interest. Within a culture killings motivated by robbery, inheritance, revenge, reward, pleasure, etc, are usually classified as murder, but would not necessarily be so considered if they were perpetrated on individuals of another culture.

Murder is often an aggressive killing performed as a supposed solution to a problem and has been practised by humans from paleontological times for a more diverse and creative range of problem-solving motives than that displayed by other species. It is seemingly enjoyed by a significant proportion of any population, either as active participants or vicariously, as evidenced by the numerous public executions and blood sports that have been organized thruout history, so that it is widely simulated as entertainment on television and in films. By this means, the extensive promotion of methods, the suggestion of suitable motives, the range of precautions needed to avoid detection and the ready availability reality-distorting drugs, is proving to be quite successful in ensuring that any individual or group contemplating such an act or acts, is fully informed and up to date.

One of the few cases where a killing for self benefit ought not be classified as murder is when it is committed in self defense. Any aspiring aggressive murderer does not usually anticipate being killed themselves by their intended victim, yet they should expect nothing less. Any misfortune that results from an attempt to murder, whether suffering from any form of delusional state or not, must attach to the perpetrator and not the intended victim. The practice of providing support and sympathy for the perpetrator at the expense of the victim is a grotesque perversion. It is to be freely admitted that it is a misfortune to suffer from any form of psychiatric or drug-induced disorders, but that misfortune must be constrained to the possessor of the misfortune and not be perversely allowed to involve other incidental sacrificial victims.