A death caused by a lawful act done under the reasonable belief that no harm was likely to result.
Accidental killing is different from Involuntary Manslaughter, which causes death by an unlawful act or a lawful act done in an unlawful way.
The Common Law of crimes distinguished two types of accidental killings: (1) accidental killings resulting from unlawful acts of violence not directed at the victim were punishable as Manslaughter (killings resulting from unlawful acts directed at the victim were punishable as murder); and (2) accidental killings resulting from lawful acts of violence were excusable as homicide by misadventure.
For example, suppose that the defendant killed an innocent bystander while carrying out an assault, Battery, or other violent crime against the intended victim. The defendant told police that he intended to injure the victim by hitting him with a club but instead struck the bystander on the skull and killed him. The defendant could be prosecuted for manslaughter under the common law of crimes.
Now suppose that the defendant was lawfully defending himself or his property from attack, and in the process killed an innocent bystander. The defendant told police that lethal force was necessary to thwart an attack upon his person, and he tried to shoot the attacker but instead killed a nearby pedestrian, who had nothing to do with the attack. The common law would have treated the bystander's death as an excusable accidental killing, so long as reasonable grounds existed for the defendant's belief that lethal force was necessary for Self-Defense.
Although most states have abolished the common law of crimes, some of the concepts underlying the common law distinctions between manslaughter and accidental killings continue to appear in statutory classifications of manslaughter.
Most states recognize at least two classes of manslaughter, voluntary and involuntary. In these states voluntary manslaughter is defined as act of murder reduced to manslaughter because of extenuating circumstances such as adequate provocation (for example, murder committed in the heat of passion) or diminished capacity. Involuntary manslaughter is defined in these states as a Homicide that is committed with criminal negligence or during the commission of a crime that is not included within the Felony-Murder Rule but for which the prosecution has no proof that the defendant intended to kill the victim or do grievous bodily harm.
Accidental killings that do not result from the defendant's criminal Negligence and do not occur during the commission of a crime are not criminal offenses in these jurisdictions. Some jurisdictions expressly classify accidental killings as excusable or justifiable homicides, such as the state of California, which provides that "[h]omicide is excusable … [w]hen committed by accident and misfortune, or in doing any other lawful act by lawful means, with usual and ordinary caution, and without any unlawful intent." CA PENAL § 195. Other states simply omit this class of homicide from their statutes defining prosecutable offenses.