How do I become a Sith?

You can't actully become a sith but you can take the dark side ending.

You do a lot of dark things, and go to the Sith Academy on Korriban. You kill both the teachers at the end of Sadow's tomb, and you become the top dog...I think. I dunno. I spared Yuthura because I'm on the Light Side of the Force. But, anyway, I suppose that's all you have to do. You get a sweet lightsaber at the end of said tomb, but you can't modify it at all, so I don't use it at all. You can't sell it, either, so I'm effectively stuck with it. Meh.

If you have the little girl on the Ebon Hawk, just go talk to her then tell her to go away, or be as mean as you can to her. You then get dark points. Do this as much as you want to become as dark as you want.

Any time.

You just choose wrong desiscions until the very ending and you choose to kill mission and zaalbar and jolee and juhani

If your on pc then you can enable cheats and turn all the way to the dark side

Wondering the same thing. I am completely dark and killed both in the tomb. However after returning to the academy the students notice their master is gone and they don't believe you when you say you killed him. Then all hell brakes loose and everyone tries to kill you. Was kinda hoping to rule the sith academy :(