Nannie Doss
Nannie Doss (November 4, 1905 – June 2, 1965) (also known as the Giggling Granny) was 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 and the most horrific of all was that she had no known motive other than she found it amusing.
This, however, was proven as being not true. She had a sort of twisted reason, to get back at life for living an unhappy childhood. That isn't to say that she didn't find it amusing, she did.