Mass murder as defined by the Cambridge Dictionary is the killing a lot of people. It usually will take place at a single location all at once or within a short period of time. The murders can be perpetrated by one person or a group of people or organization. The term mass murder is also used to describe the extermination of entire communities or groups of people based on their religion or ethnic background. Many refer to that type of extermination as genocide or a crime against humanity.
There is also a concept of mass murder called state-sponsored mass murder. That is referred to as the indiscriminate, premeditated and intentional killing of large numbers of people at the hand of members or agents of the government. This can include throwing a grenade into a prison cell or shooting of unarmed protesters.
Genocide is the most prevalent form of government-sponsored mass murder, where the systematic killing and destruction is of a national, religious or ethnic group. At the end of the 20th century the world witnessed the Srebrenica genocide and the Rwandan genocide. The killing of a political group in a country is also a form of mass murder and is referred to as political mass murder. The careful and calculated murder of prisoners or captives during a time of war by military forces is known as deliberate massacre. These types of murders have been committed extensively all through the 1900s by Nazis, the Soviet Union and the Japanese empire during World War II. In 1940 the world witnessed the massacre of Polish citizens in the Katyn Forest, the mass murder of political prisoners and the mass killing of Soviet Jews at Babi Yar.
Mass murder is not limited to the murder of people not known by the murderer, but many times includes killers of co-workers, co-students and even family and friends. There are many motives for committing mass murder, but one of the most common motives is the desire for fame and attention. America is no stranger to mass murderers, but some of the best known include Dylan Klebold, Richard Speck, Eric Harris, Charles Whitman, and Perry Smith.
When workers attack their co-workers, usually at the place of work, they are referred to as disgruntled workers. In reality they are no longer workers at the company, but ex-workers, since they have usually been removed from their job. They feel an urge to take revenge on the company, so they return to the place of work with guns and ammunition and shoot at their former colleagues. During the 1980s, in separate occurrences, two postal employees who had been fired came back and carried out massacres of their former co-workers. Since then the phrase "going postal" has become synonymous with acts perpetrated by disgruntled employees.
There have been cases where mass murders occurred during rioting in prisons. One example is the New Mexico State Penitentiary riot which broke out in February 1980 where 33 inmates were killed. Most of those inmates were actually in the Protective Custody Unit when they were murdered by other prisoners who either stabbed or axed them to death or burnt them alive.
Many serial killers or mass murderers commit their crimes because of sexual motives. They have what is termed a sexual fetish, in which they experience sexual arousal and pleasure from murdering others.
In most cases, serial murder usually involves strangers where there is no relationship between the victim and the offender. This fact makes investigating such murders very difficult, since there is no obvious connection between offender and victim, so it is difficult to find a motive. Serial murder scenes can also have strange or unusual features that can hide any identifiable motives. The same holds true for mass murder scenes, but at mass murder scenes the offenders may leave tell-tale signs which make it easier for law enforcement to catch them. This is especially true if the motive is for notoriety.