Polly Bartlett killed 22 at the family Inn in Wyoming using arsenic.

Spoiler alert: she didn't get away with it.
Quote:
In the height of the gold rush, South Pass City, Wyoming, and the surrounding mining camps were wild and dangerous.
It was not uncommon to learn that someone had died unexpectantly from everything from a bad play at a poker game to an Indian attack.
When men went missing, it was so commonplace that it took some time before it was discovered that the cause for 22 missing men was one young woman with a supply of arsenic.
James L. Sherlock had grown up in the mining town of South Pass and heard the story of the murders from his uncle and others who witnessed the wild times for themselves.
His family met many characters of South Pass, including Old Jim Bartlett, the father of Polly Bartlett, who would later become known as the "Deadly Damsel" and the "Murderess of Slaughterhouse Gulch."
Sherlock shared the story of this young serial killer in his book "South Pass and Its Tales," and claimed quite adamantly that every word is true.
Knowing the nature of just how wild that town was, the 22 murders could very well have happened just as Sherlock claims.
They also make her the worst serial killer in Wyoming state and territorial history.

Spoiler alert: she didn't get away with it.