From Peter Vronsky: In France between 1664 and 1672, the aristocratic Marie de Brinvilliers was reported to have poisoned fifty or more victims. Prior to murdering her father, who opposed her marriage, and then her two brothers to seize an inheritance, Marie experimented with poisons concocted by her lover in hospital charity wards, where she began volunteering to care for patients. She carefully observed the effects of her poison on the patients, adjusting the doses accordingly. Marie was discovered and became a fugitive until she was captured in 1676 and beheaded in Paris.
[Peter Vronsky, Female Serial Killers: How and Why Women Become Monsters, 2007, Berkley Books, p. 94]