This was one of the discussions from Wikipedia
Female serial killers
Female serial killers are rare. They tend to murder men for material gain, are usually emotionally close to their victims, and generally need to have a relationship with a person before killing them. "An analysis of 86 female serial killers from the U.S. found that the victims tended to be spouses, children or the elderly." The methods they use for murder are covert or low-profile, such as murder by poison (the preferred choice for killing). They commit killings in specific places, such as their home or a health-care facility, or at different locations within the same city or state. Other methods used by female serial killers include shootings (used by 20%), suffocation (16%), stabbing (11%), and drowning (5%). Though most female serial killers murder for money or other such material gain others do it for attention. While many female serial killers have been diagnosed with Münchausen syndrome, little research has been conducted focusing on the societal influences—particularly gender roles and expectations of women—which contribute to these women committing multiple murders. Each killer will have her own proclivities, needs and triggers, as specific reasons can only be obtained from the killer herself. "In a review of published literature on female serial murder, sexual or sadistic motives are believed to be extremely rare in female serial murderers, and psychopathic traits and histories of childhood abuse have been consistently reported in these women." On some occasions, women may be involved with a male serial killer as a part of a serial killing "team".
Kelleher and Kelleher (1998) created several categories to describe female serial killers. They used the classifications of black widow, angel of death, sexual predator, revenge, profit or crime, team killer, question of sanity, unexplained and unsolved. In using these categories, they found that most women fell into the categories of black widow and team killer. In describing murderer Stacey Castor, forensic psychiatrist Dr. James Knoll offered a psychological perspective on what defines a "black widow" type. In simple terms, he described it as a woman who kills two or more husbands or lovers for material gain. Though Castor was not officially defined as a serial killer, it is likely that she would have killed again.
A notable exception to the typical characteristics of female serial killers is Aileen Wuornos, who killed outdoors instead of at home, used a gun instead of poison, killed strangers instead of friends or family, and killed for personal gratification. The most prolific serial killer in all of history is allegedly Elizabeth Báthory. Countess Elizabeth Báthory de Ecsed (Báthory Erzsébet in Hungarian, August 17, 1560 – August 21, 1614) was a countess from the renowned Báthory family. After her husband's death, she and four collaborators were accused of torturing and killing hundreds of girls and young women, with one witness attributing to them over 600 victims, though the number for which they were convicted was 80. Elizabeth herself was neither tried nor convicted. In 1610, however, she was imprisoned in the Csejte Castle, where she remained bricked in a set of rooms until her death four years later.
An article which addressed some of the misperceptions of female criminality has appeared in the forensic literature. The Perri and Lichtenwald article addresses the current research regarding female psychopathy and includes case studies of female psychopathic killers featuring Münchausen Syndrome by Proxy, Cesarean Section Homicide, Fraud Detection Homicide, female kill teams, and a female serial killer.