MANSLAUGHTER
The law generally differentiates between levels of criminal culpability based on the mens rea, or state of mind. This is particularly true within the law of homicide, where murder requires either the intent to kill, or a state of mind called malice, or malice aforethought, which may involve an unintentional killing but with a willful disregard for life. The less serious offense of manslaughter, on the other hand, is the taking of human life but in a manner considered by law as less culpable than murder. Manslaughter is usually broken down into two distinct categories: voluntary manslaughter and involuntary manslaughter.
Voluntary manslaughter
Voluntary manslaughter cases where the defendant may have an intent to cause death or serious injury, but the potential liability for the person is mitigated by the circumstances and state of mind. The most common example is the so-called heat of passion killing, such as where the defendant is provoked into a loss of control by unexpectedly finding a spouse in the arms of a lover or witnessing an attack against his or her child.
There have been three types of voluntary manslaughter recognized in law, although they are so closely related and in many cases indistinguishable that many jurisdictions do not differentiate between them.
* Provocation. This is a killing caused by an event or situation which would probably cause a reasonable person to lose self-control and kill.
* Heat of Passion. In this situation, the actions of another cause the defendant to act in the heat of the moment and without reflection. This falls under the provocation heading.
A third type, is allowed in some but not all states. This is called "Imperfect Self-Defense". Obviously, self defense is a complete defense to any charge of murder. However, if a person acted in the honest but unreasonable belief that self defense justified the killing, many states will define this deliberate homicide committed without criminal malice, a manslaughter. The word "malice" is used in the definition of murder where the act is both an intentional killing and there is no legal excuse or mitigation. The honest belief in the need for self defense mitigates the crime so that one acts intentionally but without the legal "malice". Therefore you have an intentional killing which is unlawful but does not rise to the level of being a murder.
* Diminished Responsibility is another defence to murder that will negate the charge down. Most states require an almost complete mental breakdown to eliminate the culpable mental state of "malice". If a jurisdiction recognizes that a person can kill with justification but also without any evil intent, that jurisdiction is free to define the crime as something less than murder. Not all states do this, in many, a mental defect or even mental illness will not reduce the seriousness of the offense whatsoever. However, if a state legislature chooses, a diminished mental state may justify the finding of a lesser crime.
Insanity is a different defense as it completely negates any criminal culpability although the mental health consequence can result in as much confinement time as a murder conviction.
Involuntary manslaughter
Involuntary manslaughter, sometimes called criminally negligent homicide in the United States, Gross negligence manslaughter in the UK or culpable homicide in Scotland, occurs where there is no intention to kill or cause serious injury but death is due to recklessness or criminal negligence.
Criminal negligence
Negligence consists of conduct by an individual which is not reasonable -- that is, the individual did not act with the care and caution of a reasonable person in similar circumstances. This "reasonable person" is fictitious, of course, but reflects the standard of conduct which society wishes to impose. Violation of this standard may lead to civil liability for the consequences of the negligent behavior.
Negligence rises to the level of criminal negligence where the conduct reaches a higher degree of carelessness or inattention, perhaps to the point of indifference.
Recklessness
Recklessness or willful blindness is defined as a wanton disregard for the known dangers of a particular situation. An example of this would be a defendant throwing a brick off a bridge into vehicular traffic below. There exists no intent to kill, consequently a resulting death may not be considered murder. However, the conduct is probably reckless, sometimes used interchangeably with criminally negligent, which may subject the principal to prosecution for involuntary manslaughter: the individual was aware of the risk of injury to others and willfuly disregarded it.
In many jurisdictions, such as in California, if the unintentional conduct amounts to such gross negligence as to amount to a willful or depraved indifference to human life, the mens rea may be considered to constitute malice. In such a case, the charged offense may be murder, often characterized as second degree murder.
In some jurisdictions, such as Victoria, British Columbia, recklessness is sufficient mens rea to justify a conviction for murder.
Vehicular or Intoxication manslaughter
Vehicular manslaughter is a kind of misdemeanor manslaughter, which holds persons liable for any death that occurs because of criminal negligence or a violation of traffic safety laws. A common use of the vehicular manslaughter laws involves prosecution for a death caused by driving under the influence (determined by excessive blood alcohol content levels set by individual states), although an independent infraction or negligence is usually also required.
In some US states, such as Texas, Intoxication Manslaughter is a distinctly defined offense. A person commits intoxication manslaughter if he or she operates a motor vehicle in a public place, operates an aircraft, a watercraft, or an amusement ride, or assembles a mobile amusement ride; and is intoxicated and by reason of that intoxication causes the death of another by accident or mistake.
Intoxication manslaughter, vehicular manslaughter and other similar offenses require a lesser mens rea than other manslaughter offenses. Furthermore, the fact that the defendant is entitled to use the alcohol, controlled substance, drug, dangerous drug, or other substance is not a defense. [2] For example, in Texas, to prove intoxication manslaugher it is not necessary to prove the person was negligent in causing the death of another, only that they were intoxicated and operated a motor vehicle and someone died.
Misdemeanor manslaughter
In the United States, this is a lesser version of felony murder and covers a person who causes the death of another while committing a misdemeanor -- that is, a violation of law that does not rise to the level of a felony. This may automatically lead to a conviction for the homicide if the misdemeanor involved a law designed to protect human life. Many violations of safety laws are infractions, meaning that a person can be convicted regardless of mens rea.