When someone is killed lawfully by someone who did not have “malice aforethought,” as it is known, the killer is charged with manslaughter. Manslaughter is separate from murder, recognizing that people can occasionally kill each other without meaning to, and it tends to carry a reduced sentence, depending on the circumstances in which the killing occurred. When someone kills in self defense and his or her actions are ruled justified, this is an entirely different legal situation, because the death is viewed as lawful, though unfortunate.
There are two types of manslaughter: voluntary and involuntary. Voluntary manslaughter occurs when someone commits a crime due to provocation. For example, if two men get in a fight outside a bar and one of the men accidentally kills the other, he will be charged with manslaughter, because he has caused the death of a human being, but he didn't mean to. Likewise, a burglar who is surprised by a security guard and accidentally kills the security guard would be charged with manslaughter, unless he committed the killing with a weapon such as a gun, which would suggest some malice aforethought.
The line between murder and voluntary manslaughter can sometimes be very fine, and it is possible for charges to be changed to reflect new information in a case. Small distinctions can upgrade a charge to murder. For example, if the accused started a fight with the goal of provoking the victim and the victim was killed in the course of the fight, this might be considered murder, especially if the accused and the victim had a past history of animosity.
Involuntary manslaughter occurs as a result of negligence. The killer was not provoked, and did not mean to kill anyone, but his or her negligent actions led to a death. A classic example of this form is vehicular manslaughter, in which someone's reckless driving leads to a death. If someone can prove that the negligence was intentional, the charge may be upgraded to murder. In some regions, someone with a history of driving under the influence (DUI) convictions may be charged with murder for a death which occurs as a result of drinking and driving, rather than manslaughter.
The penalty for this crime is usually some jail time, and sometimes the sentence can be reduced with public service or cooperation; for example, someone who kills someone else by driving recklessly could appear in a public service announcement talking about the consequences of reckless driving, or travel to schools to warn teens about the dangers of driving dangerously. The killer may also be subject to a civil suit from the victim's family which attempts to recover damages from the killer.