Locusta was one of the first recorded professional chemists. She was employed by several royal Romans, and even established a school for other chemists. Here’s why it was best not to piss off either her or her students.
Source: Badass Historical Chemists: Locusta
Some people believe she was the first recorded serial killer. Not everyone she poisoned was for profit. It’s a toss up as to whether she poisoned them for fun or for practice.
My Warfarin Diet 