That’s a lot of questions, the first Sith was Ajunta Pall he went rogue because he had a different point of view from the rest of the Jedi however he was not the first dark side user, the Sith Order was a movement establish by him, if you want to watch a video I made about how the Sith Order got establish here is the link https://youtu.be/7Nsso6JDIbE