Angel of Mercy
Despite their prevalence in popular culture, serial killers are a statistical rarity. Serial murder (defined as three or more victims with a significant time gap between them) is extremely rare. Studies suggest that less than 1% of murders meet this definition each year.
Serial murder perpetrated by women is an extremely small slice of this already miniscule piece of pie. Less than 15% of serial murder is done by women. It is one area where it is good that men and women have not achieved parity, I suppose.
Almost all female serial killers fall into one of three categories: black widows (those that serially murder their partners, usually by poison), team killers (women who join with a partner - almost always a man - to kill), and angels of mercy (caregivers who murder those in their care). The final category is the most common.
Angel of mercy serial killers are among the most vexing and traumatic because they seem to be providing care for their victim. Frequently these women are described in glowing terms by colleagues, friends, and clients. The families of their victims often heap praise on them and see them as lifesavers when they are the exact opposite. It is by building this trust that these killers are able to gain access to their victims.
The results are predictably devastating.