Koyi Mateil
Always love to see some moral complexity! So in terms of Sith factions there are two main ones right now, although I encourage you to check out the other factions and see what they can offer as well.
There is the
Brotherhood of the Maw which is the biggest Dark Side oriented faction as of right now, engaged in some heavy warfare against a couple other galactic powers. They are a good choice although they are definitely heavy on the evil factor, but of course that is not always a bad thing when it comes to crafting some interesting stories and dynamics.
Then you have the faction that by boy belongs to, the
Sith Order. Which is pretty new in terms of a place on the galactic stage, but that leaves plenty of room for development as well. Mainly our focus is spreading our influence and managing internal warlords who challenge one another for total control of the wider order based on their personal views and philosophies regarding the Sith.
Of course these two are just the main ones and many other factions have Sith in them as well, so don't feel constrained by only these two options!