As a writer I can't help having concepts in mind when I make a character. In the case of my Sith Warrior (and Knight, both having very open backgrounds) I simply borrowed from my own stories, adapted a personality to fit Star Wars, and made choices based on what she would do.
For me the so-called "Light Side" path was better, satisfying. For you it might be "Dark" or somewhere in between. The story progresses towards the same goal. We just get to influence it along the way.
My character is a martial warrior and strategist, a female Alexander the Great. Most Sith, to her, are self-destructive slaves to passion, not masters of it. Jedi are naive children, fearful of the dark, living in constant self-denial. Two extremes, doomed to fail. Fear is a short-term means of control, has a shorter breaking point. Adoration - even love - is longer-lasting and capable of accomplishing greater things (those who love you will often go to greater lengths, sacrifice more than those who fear you). The Republic is weak, democracy a great idea that will never work in practice. Imperialism is the only solution, an Empire ruled by her. She's prideful, vain, and despises Baras for sidetracking her plans (I took every opportunity to belittle him, all the ones that came across as condescending).
I had no idea how the story would go, just kept the above in mine as I went. It actually made some of the Light Side choices seem sinister, even cruel. And there was nothing weak, naive, or self-sacrificing about the way they played out. She's the kind of "evil" that does not believe she is evil, remembered by history as a savior or a tyrant depending on the point of view. I did take a Dark Side choice or two but only if they made sense to the character I was playing. One particular Light choice at the end followed by the Dark option that popped up next came out sounding so smug, condescending, and sadistic I had chills, sat here for a bit thinking, "That was perfect!"
Again the whole point is: what works for me or someone else might not work for you. Play it the way you feel it should be played.