Anna Zwanziger: 

In the early years of the 19th century, a German 
widow named Anna Zwanziger, who grotesquely ugly, employed herself 
out as a housekeeper and cook to successful middle-aged judges. 
In her fantasies, Anna hoped that one of these judges would be 
so impressed and dependent on her domestic skills that he would 
propose to her. However, there was a flaw in Anna's plan, each 
man was already married or engaged to another woman. After much 
thinking, Anna devised a solution: she poisoned two of the women 
with arsenic. She also poisoned one of the judges, several servants, 
and a baby. Just before she was beheaded in 1811, Anna told the 
prison gaurds that "It is perhaps better for the community that 
I should die, as it would be impossible for me to stop posioning 
people." 

http://members.tripod.com/~dark_star_3/womenkillers.html
When Zwanziger confessed she told the police that she trembled with 
delight when she was handling arsenic.

http://www.lostminds.co.uk/html/body_serial_killers.html
The baby died, after eating a biscuit soaked in arsenic-spiked milk.

http://asylumeclectica.com/morbid/archives/morb0501.htm
Another notorious German poisoner was Anna Zwanziger (born 1760), a Bavarian
cook, who, according to this Web site: "poisoned two of her employers and
numerous of their dinner guests. Suspected, she fled her final job, but not 
before leaving large amounts of arsenic in the coffee, salt, and sugar jars 
and a dose in the baby's biscuits."

http://www.planetpeschel.com/Sayers/poison.htm