12. Jane Toppan
This loathsome woman went by the nickname Jolly Jane. She also had a couple dozen kills to her name, which she openly boasted about. Something about harming helpless people gave her a sense of power. Needless to say, she was far from jolly.
To make matters worse, Toppan was a nurse, which meant she had access to numerous helpless people daily. Eventually, she was arrested in 1901, at which point she openly confessed to 33 murders. However, officials had reasons to believe she was severely mentally ill. As a result, Toppan was found not guilty by reason of insanity and committed to a mental institution for life.