13. Mary Ann Cotton
Mary Ann Cotton’s kill count is somewhat dubious. What is known, however, is that she killed multiple husbands and many of her own children. She was also very sly about how she went about her dirty business, although the pain her victims faced was far from subtle. All the victims were related to Cotton in some way.
Cotton had several husbands over the course of her life, four total. Of those four, she was convicted of killing three of them. She is also said to be responsible for the deaths of a dozen of her own children. Cotton was tried in court after she being accused of poisoning her stepson with arsenic. She was hanged for her crimes.