Caroline Grills -- Australia

Described as a kindly old lady, at 63 years of age she faced four 
charges of murder and two of attempted murder by the administration 
of the metallic poison thallium. While there was the motive of gain, 
she had a clear desire to exercise the power of life and death over 
her victims. Known affectionately as "Aunty Thally", she was found 
guilty of one attempted murder charge and sentenced to life imprisonment.  

http://www.redbackwebs.com.au/cufflinks/police/serial.html#Grills

http://www.fireflybooks.com/Publicity/books/4839.html