Until Aileeen Wuornos came along, the term "female serial killer" was generally believed to be an oxymoron, in spite of the fact that some of our fairer citizens have consistently shown us that, when it comes to murder, a woman can compete with any man. Of course, part of this lack of awareness was caused by earlier definitions of serial murder which commonly assumed the perpetrator would be a man; as late as 1998, Roy Hazelwood of the FBI was quoted at a conferences as saying "there are no female serial killers." Even when the lethality of a femme fatale is recognized, she is most often portrayed as a victim manipulated by an evil male.
It's true that there are far less female serial murderers than there are males (although percentage-wise, there are more female serial murderers (15 percent in comparison to 85 percent males) than there are one-time killers (90 percent men; 10 percent women). It's also true that you aren't as likely to see a woman torturing her victim or having sex with a dead corpse, the kinds of gory details that make headlines and sell movies. In fact, sex and pleasure are likely to be much farther down on the motivation list for female serial killers than men; we females tend to take a more pragmatic approach to killing people off. Female serial killers kill for profit and power.