Intense study in the field of serial murder has resulted in two ways of classifying serial killers: one based on motive and one based on organizational and social patterns. The motive method is called Holmes typology, for Ronald M. and Stephen T. Holmes, authors of numerous textbooks on serial murder and violent crime. Not every serial killer falls into a single type, and many are more than one type. Neither of these classifications explain what might actually lead someone to become a serial killer (more on this later). There is not enough scientific data upon which to base these classifications, either -- they are based on anecdotal and interview data. Critics of the Holmes typology point to this as a flaw, but many investigators still find the method useful when studying serial murder.
According to Holmes typology, serial killers can be act-focused (who kill quickly), or process-focused (who kill slowly). For act-focused killers, killing is simply about the act itself. Within this group, there are two different types: the visionary and the missionary. The visionary murders because he hears voices or has visions that direct him to do so. The missionary murders because he believes that he is meant to get rid of a particular group of people.
Process-focused serial killers get enjoyment from torture and the slow death of their victims. These include three different types of hedonists -- lust, thrill and gain -- and power-seeking killers. Lust killers derive sexual pleasure from killing. Thrill killers get a "kick" from it. Gain killers murder because they believe they will profit in some way. Power killers wish to "play God" or be in charge of life and death.