Mass murder is commonly defined as the murder of 4 or more people in the one incident.

But this is a very simplistic empirical definition that doesn't distinguish the military or terrorist mass murder from the so called random lone gunman one. And excludes many cases that fit that type perfectly (except for high bodycount)

So, in compiling the collection of mass murderers here, I have made certain decisions about what to include or exclude based on my own personal classification of the incident in question. This process has not been systematic in terms of using body count, method, motive or any other such aspect, but rather, simply what I feel is needed to construct an idea of what the lone mass murderer is.

When I say lone, I mean it more than just in the sense of the number of actual perpetrators. Social support for the killing is also a factor. For instance, Baruch Goldstein, who killed around 30 at a Mosque, has been left out because he was doing essentially what a suicide bomber there does. Killing the enemy. It was a civilian attacking civilian targets, but was an act greatly endorsed by his people. The type of lone killer I am referring to has no public support from anyone in mainstream society, at least not for the act of killing itself.

Among other cases I have excluded are those that were commissioned as a secondary to another crime. This includes armed robbery where the robbers lost it and killed their hostages, or break and enters where the burglar unwittingly got caught and went berserk. Similarly I have excluded those mass murders where profit, terrorism or extortion has been the primary motive. A few exceptions of family mass murder have however been included, that may have been profit motivated to a degree. Drunk Russian soldiers, who have gone on many a rampage, I have excluded for the moment, but may include at a later date. They sit somewhere between the war inspired rampage and the workplace one, since many are not in combat. We'll see.

Arson as a means of killing I have typically left out, although I have included the more notable cases, such as Julio Gonzalez who with $1 worth or petrol killed 87 people after a fight with his ex. Robert Segee, whose arson attack at a circus in 1944 left 169 people dead has been included despite his being more a serial arsonist who got unlucky than a mass killer, simply to illustrate the potential of other means of killing besides guns.

However, what has been included is primarily the lone nut with a gun and a grudge, and the family mass murderer. Initially I was going to leave out the family killers, because they are quite a different breed to the classic mass murderer. As I said in the classification section, a family mass murderer is typically just a spouse killer with a lot of kids. But then I decided there were so many that they needed to be included to make a point. If you are going to be a victim of a massacre, it's just as likely to happen in your own house.

I also included killers who did not make it past the technical definition of 4 fatalities, if I felt they fit into the 'spirit of mass murder'. That is, they had the intention to kill dozens, and made the effort, but on the day their aim was off and they wounded more than they killed.

Another category I have included, with some reluctance I must admit, has been postal workers. There have been several genuine mass murdering types, who started off the whole postal thing, but since then it's been mostly hype. Any connection between postal workers and violence has been jumped on in the media, even where we're talking mundane, everyday violence experienced all over the place. For example, there is a more than subtle difference between shooting a single hated supervisor at work after being fired, and shooting everyone in sight. But I've thrown in a few of these anyway, the ones that were perhaps most like the mass killer, even if it’s a bit debatable.