There are 4 types.

Visionary : Kills because they have a psychosis which causes them to believe supernatural power are commanding them. Very disorganized and random.

Missionary : Kills to eliminate a group of people deemed undesirable. Kills very quickly with non-sexual motive.

Hedonist : Lives a deviant lifestyle (full of lust, crime, etc.) and the deviant lifestyle becomes intertwined with murder. Further subdivided into lust killers (sex addicts), thrill killers (compulsive risk takers), and profit oriented (hitmen).

Power/Control : A Jekyll and Hyde killer who wants to totally dominate the victim, and plans crime in extreme detail. Because he wants full control when he takes the mask off (and to avoid detection), he lives his life tricking people into believing that he is a regular guy.

http://en.wikipedia.org/wiki/Serial_killer#Motives

I don't believe I'd include "hitmen" on the list, and if I read the Wikipedia article correctly it specifically excludes them.

The Syndicate-style hitman kills only those people whom he is ordered to by his own superiors, operating in very nearly a military-precision chain-of-command. The one thing he's NOT is a mad-dog loose-cannon killer out roaming the streets. The one thing the Syndicate does NOT want is CRAZY enforcers. (It brings the cops and the reporters around too much.)

That in no way excuses what the hitman does, please understand. It's still cold-blooded murder.

Holmes and Deburger originally agreed with you, as did I. They changed their minds in later books. So did I.

What they said was that it would be a mistake to assume that there is a sexual motive for all serial killings. I agree with that. A serial killer can really only be defined as a repeat killer. If you get picky with the definition, it would just become very confusing.