Through passion, I gain strength.
Through strength, I gain power.
Through power, I gain victory.
Through victory, my chains are broken.
The Force shall free me.
Although this statement is considered the Sith Code, the guide by which all Sith live their lives, it's more a counter to the Jedi Code than anything. Because the first Sith Lords as we know them in Star Wars: The Old Republic were former Jedi, it only makes sense that they would create a code intended to be the antithesis of the Jedi's central theme. Not all Sith follow the Sith code, but most believe in its credence.
If the Sith Code isn't central to the Sith beliefs, what is? That's the interesting thing: There doesn't seem to be a central theme other than to be the opposite of the Jedi. Some Sith believe in an Empire. Some Sith believe there can be only two. Yet the Sith existed long before there was a Sith Code. Before there was a Sith Order, there was the Sith Empire on Ziost and Korriban, and that is where the SWTOR Sith come from. That is where my Sith characters come from. I don't believe I have all the philosophical answers, and clearly, my way is not the only way to roleplay a Sith, but perhaps I can give you a launchpad to start your own storylines.