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.
Although Locusta was one of history's great chemists, and lived in the relatively well-recorded first century AD, we don't know much about her work. This is because her focus, practically-speaking, was poisons. And she was good at her job. Well-known, to those who knew who to ask, she made a great connection when she helped a noble woman named Agrippina, who had a very inconvenient husband. Locusta helped her dispatch him, and go on to a very convenient one — Claudius, the emperor of Rome and Agrippina's uncle. When he eventually also became inconvenient (by possibly leaving the rule of Rome to his biological son, Britannicus, rather than Agrippina's son, Nero) Locusta supplied a dish of poisoned mushrooms.
Locusta then switched bosses, working for Nero, who had her get rid of Britannicus. At this point, it was obvious to the royals that it paid to have the best poisoner in Rome on their side. With Nero's help, Locusta set up a school for poisoners. At the school, Locusta and her students made new poisons, gauged their strength by testing them on humans and animals, and did research on how to detect poisons in food, wine, or embedded in material.
It's a shame that no records of Locusta's research survive, as they certainly would make interesting reading. To this day, scholars try to figure out what poisons she used, and which rich and famous Romans fell victim to them. Some people believe that she killed Britannicus with cyanide, but beyond that the best anyone can do is speculate that she used plant-based poisons, instead of the more well-known arsenic.
Alas, Locusta's career as a semi-public poisoner-for-hire was doomed. She'd been condemned and awaiting execution several times, only to be saved by some royal who had a use for her. When Nero fell, so did she. Nero's successor, Galba, sentenced her to death, and this time it took. She died in approximately 68 AD.