Murder is the killing of another person without justification or valid excuse, and it is especially the unlawful killing of another person with malice aforethought. This state of mind may, depending upon the jurisdiction, distinguish murder from other forms of unlawful homicide, such as manslaughter. In some U.S. states, laws regarding murder are determined by the Model Penal Code.
Most societies, from ancient to modern, have considered murder a very serious crime deserving harsh punishment for purposes of retribution, deterrence, rehabilitation, or incapacitation. There are many reasons why murder has been criminalized, including its costs to society as well as being considered intrinsically wrong. For example, murder may be considered intrinsically wrong because it violates a right to life, or objectifies the victim, or is oppressive; murder may be costly to society by undermining law and order, by squandering potential accomplishments of the victims, by risking escalation of violence, or by spreading fear and grief.
In most countries, a person convicted of murder is typically given a long prison sentence, possibly a life sentence where permitted. In some countries, the death penalty may be imposed for such an act – though this practice is becoming less common.