Good grief, what a cheerful subject! Speculating on what the criteria are for a mass murder, as opposed to a spree killer. Jeez.
Murder has been around as long as people have walked the earth, but I have never understood the compulsion to kill numerous people at one time. I've been angry enough to want to kill someone -- some *one.* One person. But never so angry that I would want many people to die.
I guess the capability to commit mass murder is an indication something is seriously mentally messed up with a person. It has to be a pathological mental disorder that enables someone to commit those kinds of crimes.