Mary Ann Cotton -- england, late 19th century

Over the course of 28 years, Mary Cotton lost 7 children, 
3 husbands (one surviving because abandoned), 4 stepchildren,
a sister-in-law, a mother, and a boarder -- all to arsenic.
She even reportedly poisoned a neighbor's herd of pigs.

http://www.crimelibrary.com/serial7/widow/4.htm

http://www.crimelibrary.com/serial10/cotton/6.htm

http://www.geocities.com/Area51/Shadowlands/4077/cotton.html

http://www.murderuk.com/serialkillers/maryanncotton.htm

http://www.fortunecity.com/roswell/hammer/73/cotton.html