Where manslaughter comes from.

At early common law only those homicides committed in the enforcement of justice were considered justifiable; all others were deemed unlawful and were punished by death. Gradually, however, the severity of the common-law punishment for homicide abated. Between the 13th and 16th centuries the class of justifiable homicides expanded to include, for example, accidental homicides and those committed in self-defense. Concurrently, the widespread use of capital punishment was ameliorated further by extension of the ecclesiastic jurisdiction. Almost any person able to read was eligible for 'benefit of clergy,' a procedural device that effected a transfer from the secular to the ecclesiastic jurisdiction. And under ecclesiastic law a person who committed an unlawful homicide was not executed; instead he received a one-year sentence, had his thumb branded and was required to forfeit has goods. At the turn of the 16th century, English rulers, concerned with the accretion of ecclesiastic jurisdiction at the expense of the secular, enacted a series of statutes eliminating the benefit of clergy in all cases of 'murder of malice prepensed.' Unlawful homicides that were committed without such malice were designated 'manslaughter,' and their perpetrators remained eligible for the benefit of clergy. Even after ecclesiastic jurisdiction was eliminated for all secular offenses the distinction between murder and manslaughter persisted. Mullaney v. Wilbur, 421 U.S. at 692-93.