Most prolific serial killer (female)

The most prolific female serial killer, based on the number of confirmed victims, is American nurse Jane Toppan (b. Honora Kelley). She confessed to having killed 31 people over the course of her nursing career, which began in 1885 and ended with her arrest in 1901. She was tried on 12 counts in 1902, but found not guilty by reason of insanity and committed to a mental asylum.

Honora Kelley was born in 1854. Her mother died of tuberculosis when she was very young and her father, reportedly an abusive and likely mentally ill alcoholic, surrendered her and her sister to an orphanage in 1860. At the age of eight, she was placed in the home of the Toppan family in Lowell, Massachusetts, as an indentured servant girl. There, she seems to have been treated as part of the family, and took on the name "Jane Toppan", though she was never formally adopted.

Toppan trained as a nurse at Cambridge Hospital in Massachusetts and while she was outwardly cheery and diligent, she soon developed a morbid fascination with the effects of the drugs morphine, strychnine and atropine. She would experiment on her patients, raising the dose to make them drift in and out of consciousness or to slow their respiratory systems until they were at the brink of of death. It is thought that she likely started killing during this period, and while she was never caught in the act, her recklessness with dosages alarmed her colleagues. Toppan's hospital nursing career ended in 1890 with her being fired from her job at Massachusetts General Hospital.

She continued to work as a private practice nurse and carer, however, and became increasingly bold in her murders. The series of killings that would lead to her arrest started in 1895, with the poisoning of the 83-year-old Israel Dunham and his wife, Lovely Dunham. She went on to kill another ten people, either patients or the family of patients, before a toxicology exam revealed her crimes.

She plead guilty to 12 counts of murder at her trial, but disputed the characterization of herself as insane. She maintained that she was of sound mind in the belief that she was more likely to be one day offered parole from prison than release from a mental institution. The jury, however, found her not guilty by reason of insanity and she was committed for life to Taunton Insane Hospital. She died there in 1939 at the age of 83.

Though the Hungarian “Blood Countess” Erzsébet (or Elizabeth) Bàthory (1560–1614) is alleged to have tortured and murdered as many as 650 victims, her life history is so shrouded in legend that it is impossible to separate fact from fiction.