Nannie Doss (also known as the Giggling Granny) is a serial-killer who managed to poison her entire family with cyanide before gleefully admitting to all her crimes to police, laughing and earning her nickname.
By the time she was finally caught, she'd killed her mother, two sisters, two daughters, a nephew, a grandson, and four husbands - most horrific of all was that she had no known motive other than she found it amusing.