"My own ambition is simply to kill as many people as humanly possible. Now that excites me."
A Mass Murderer is a person who murders four or more people in one place and time. The most common incidents of mass murder include school shootings, mass shootings and bombings. Mass murderers differ from serial killers, who kill people over long periods of time. Although a character can be both, if say, they murdered different groups of people over a long period of time.
Mass murderers usually operate alone. Anger, resentment and malice build up in them until they explode in solitary savagery.