manslaughter is the crime of killing a person. when a person is killed, lawyers also look at the reasons, and motives why this person was killed. if the death of a person was not on purpose, but the killer did something that resulted in the death of the another person, this is usually called manslaughter, in english. it is less serious than if a person was killed on purpose (sometimes with planning involved), which is called murder, in english. killing someone in self-defence may be manslaughter. depending on the circumstances, killing in self-defence may be no crime at all. negligent homicide is an even lesser crime.