The first Sith, IIRC, was a "dark Jedi" named Aka Sith who disagreed with the Supreme Jedi Council's philosophes, and ideals that balancing harmony, focus, and intelligence were the only ways to access the Force, but also through extreme emotions like anger, rage, hate, and fear. The first ancient Siths were a far, bigger, more existential threat to the Old Republic and the Jedi than those weird, strange, covert hidden later Siths like Palpatine, Darth Maul, and even Count Dooku. There was a period of time in the Star Wars universe about 10,000 years before the events of Clone Wars, A New Hope where the Sith order was more loud and proud about their existence, and they engaged in a violent, bloody civil war against the Old Republic and their Jedi allies, a conflict Republic won because the Sith order were a very chaotic, extremely power-hungry, violently competitive hierarchy that fought as much among themselves as their Jedi enemies. That is really why they adopted the Rule of Two (master/apprentice) relationship. I know they made a very good video game about these events, but a really well-written, produced TV series going more in-depth and explaining the power struggles in this era would be interesting.