Homicide is the act of one human killing another.  A homicide requires only a volitional act by another person that results in death, and thus a homicide may result from accidental, reckless or negligent acts even if there is no intent to cause harm.  Homicides can be divided into many overlapping legal categories, including murder, manslaughter, justifiable homicide, killing in war, euthanasia, and capital punishment, depending on the circumstances of the death. These different types of homicides are often treated very differently in human societies ; some are considered crimes, while others are permitted or even ordered by the legal system.
Criminal homicide takes many forms including accidental or purposeful murder. Criminal homicide is divided into two broad categories, murder and manslaughter, based upon the state of mind and intent of the person who commits the homicide. 
Murder is the most serious crime that can be charged following a homicide.  In many jurisdictions, homicide may be punished by life in prison or even capital punishment.  Although categories of murder can vary by jurisdiction, murder charges fall under two broad categories:
- First degree murder : the premeditated, unlawful, intentional killing of another person; and
- Second degree murder : The intentional, unlawful killing of another person, but without any premeditation.
In some jurisdictions, a homicide that occurs during the commission of a dangerous crime may constitute murder, regardless of the actor's intent to commit homicide. In the United States, this is known as the felony murder rule.  In simple terms, under the felony murder rule a person who commits a felony may be guilty of murder if someone dies as a result of the commission of the crime, including the victim of the felony, a bystander or a co-felon, regardless their intent—or lack thereof—to kill, and even when the death results from the actions of a co-defendant or third party who is reacting to the crime. 
Manslaughter is a form of homicide in which the person who commits the homicide either does not intend to kill the victim, or kills the victim as the result of circumstances that would cause a reasonable person to become emotionally or mentally disturbed to the point of potentially losing control of their actions.  The penalty for manslaughter is normally less than the penalty for murder. The two broad categories of manslaughter are:  
- Voluntary manslaughter : the intentional, unpremeditated killing of another person, but without premeditation and as the result of a disturbed state of mind, or heat of passion .
- Involuntary manslaughter : the unintentional killing of another person through an act of recklessness that shows indifference to the lives and safety of others, or an act of negligence that could reasonably be foreseen to result in death. The act that results in death may be intentional, such as pushing somebody in anger, but their death (such as by their subsequently falling, striking their head, and suffering a lethal head injury) is not.
Another form of manslaughter in some jurisdictions is constructive manslaughter , which may be charged in the event that a person causes a death without intention, but as the result of violating an important safety law or regulation. 
Defenses to homicide
Not all homicides are crimes, or subject to criminal prosecution.  Some are legally privileged, meaning that they are not criminal acts at all. Others may occur under circumstances that provide the defendant with a full or partial defense to criminal prosecution. Common defenses include:
- Self-defense : while most homicides by civilians are criminally prosecutable, a right of self-defense (often including the right to defend others)  is widely recognized, including, in dire circumstances, the use of deadly force. 
- Mental incapacity : A defendant may attempt to prove that he or she is not criminally responsible for a homicide due to a mental disorder.
if the defendant in a capital case is sufficiently mentally disabled in the United States they cannot be executed. Instead, the individual is placed under the category of “ insane ”.
- Justifiable homicide or Privilege : Due to the circumstances, although a homicide occurs, the act of killing is not unlawful. For example, a killing on the battlefield during war is normally lawful, or a police officer may shoot a dangerous suspect in order to protect the officer's own life or the lives and safety of others. This privilege also extends itself to "pattern or practice"predetermined actions or events committed by individuals in gangs, hate groups, or domestic terrorist organizations that federal court has already proven are targeting individuals because of their ancestral origin, religious beliefs, nationality, race, color, creed, sexual orientation, age, disability, political beliefs, or ethnic group.
The defense in a homicide case may attempt to present evidence of the defendant's character, to try to prove that the defendant had a history of violence or of making threats of violence that suggest a violent character.   The goal of presenting character evidence about the victim may be to make more plausible a claim of self-defense,  or in the hope of accomplishing jury nullification in which a jury acquits a guilty defendant despite its belief that the defendant committed a criminal act. 
Homicides may also be non-criminal when conducted with the sanction of the state. The most obvious examples are capital punishment, in which the state punishes a criminal with death. Homicides committed in action during war are usually not subject to criminal prosecution either.
A 2011 study by the United Nations Office on Drugs and Crime brought together a wide variety of data sources to create a worldwide picture of trends and developments.  Sources included multiple agencies and field offices of the United Nations, the World Health Organization, and national and international sources from 207 countries.
The report estimated that in 2010, the total number of homicides globally was 468,000. More than a third (36%) occurred in Africa, 31% in the Americas, 27% in Asia, 5% in Europe and 1% in Oceania. Since 1995, the homicide rate has been falling in Europe, North America, and Asia, but has risen to a near “crisis point” in Central America and the Caribbean. Of all homicides worldwide, 82% of the victims were men, and 18% were women.  On a per-capita scaled level, "the homicide rate in Africa and the Americas (at 17 and 16 per 100,000 population, respectively) is more than double the global average (6.9 per 100,000), whereas in Asia, Europe and Oceania (between 3 and 4 per 100,000) it is roughly half". 
UNODC, in its 2013 global report, estimated the total number of homicides worldwide dropped to 437,000 in 2012. Americas accounted for 36% of all homicides globally, Africa 21%, Asia 38%, Europe 5% and Oceania 0.3%.  The world's average homicide rate stood at 6.2 per 100,000 population in 2012, but Southern Africa region and Central America have intentional homicide rates four times higher than the world average. They are the most violent regions globally, outside of regions experiencing wars and religious or sociopolitical terrorism.  Asia exclusive of West Asia and Central Asia, Western Europe, Northern Europe, as well as Oceania had the lowest homicide rates in the world. About 41% of the homicides worldwide occurred in 2012 with the use of guns, 24% with sharp objects such as knife, and 35% by other means such as poison. The global conviction rate for the crime of intentional homicide in 2012 was 43%. 
[W]here homicide rates are high and firearms and organized crime in the form of drug trafficking play a substantial role, 1 in 50 men aged 20 will be murdered before they reach the age of 31. At the other, the probability of such an occurrence is up to 400 times lower.
[H]omicide is much more common in countries with low levels of human development, high levels of income inequality and weak rule of law than in more equitable societies, where socio-economic stability seems to be something of an antidote to homicide.Women murdered by their past or present male partner make up the vast majority of [female] victims.