Homicide is the killing of one person by another. Not every killing is criminal. When the law permits one person to kill another—for example, a soldier killing an enemy on the battlefield during war, or a killing in self-defense—the death is considered the result of justifiable homicide. An excusable homicide, by contrast, is one in which death results from an accident in which the killer is not at fault.
All other homicides are criminal. The most severely punished form is murder, defined as homicide committed with “malice aforethought.” This is a term with a very long history. Boiled down to its essentials, it means that the defendant had the intent to kill. A killing need not be premeditated for any long period of time; the premeditation might be quite sudden, as in a bar fight that escalates in that moment when one of the fighters reaches for a knife with the intent to kill.
Sometimes a homicide can be murder even if there is no intent to kill; an intent to inflict great bodily harm can be murder if the result is the death of another person. A killing that takes place while a felony (such as armed robbery) is being committed is also murder, whether or not the killer intended any harm. This is the so-called felony murder rule. Examples are the accidental discharge of a gun that kills an innocent bystander or the asphyxiation death of a fireman from smoke resulting from a fire set by an arsonist. The felony murder rule is more significant than it sounds, because it also applies to the accomplices of one who does the killing. Thus the driver of a getaway car stationed a block away from the scene of the robbery can be convicted of murder if a gun accidentally fires during the robbery and someone is killed. Manslaughter is an act of killing that does not amount to murder. Voluntary manslaughter is an intentional killing, but one carried out in the “sudden heat of passion” as the result of some provocation. An example is a fight that gets out of hand. Involuntary manslaughter entails a lesser degree of willfulness; it usually occurs when someone has taken a reckless action that results in death (e.g., a death resulting from a traffic accident in which one driver recklessly runs a red light).