A far as we know, the Sith started as the dominant species on the planets Korriban and Ziost. I'm pretty sure that the Rakata had something to do with the species' attachment to the Dark Side of the Force, but whether or not that is true, we do know that the barbaric Sith led by King (Sith'ari) Adas were one of the very few groups able to drive off the Infinite Empire. But when the Jedi Exiles landed on Korriban a few thousand years later, they were able to subjugate the whole species. And using Dark-Side alchemy, the human Exiles (aka the first Dark Lords of the Sith) mingled their bloodline with the red-skinned Sith species. As generations passed, it became a symbol of pride to display evidence of the Dark Lords in appearance, such as five fingers instead of four.
When Naga Sadow became the Dark Lord of the Sith after Marka Ragnos, the Republic invaded and nearly wiped out the species. However, one Sith, who was born Tenebrae and later named Lord Vitiate by Marka Ragnos himself, was able to save the Sith Empire by retreating to Dromund Kaas and living in exile for a few thousand years until the Sith's resurgence about 30 years before the start of Star Wars: The Old Republic.
It's easy to write off the Sith as evil because they follow the Dark Side of the Force. Yet I don't know anyone who actually considers himself evil. I suspect that most people who do "evil" acts do them under the delusion of the greater good, and whether it actually is the greater good or not is up to history. The truth is that Sith attempt to bend the Force to their will, therefore creating the Dark Side of the Force. Bending the Force to one's will is not inherently evil; if you bend the Force to your will to save someone, is that evil? Sure, maybe it was the will of the Force for that person to die, but it doesn't make the act of saving him evil. And that is how the "dark path" starts.
I interpret the Force as linear -- neither good nor evil, just a neutral power to be used or followed with the purpose of keeping the universe in order. And there is an order or balance to the Force: When everything flows with the will of the Force, then we have the Light Side. However, when the Force is subjugated to the will of the user, then you have the Dark Side. And that is the focus of the Sith religion: gaining power and strength by bending the Force to their will.
I don't expect everyone to follow my character's belief system, but I'll lay out one of the biggest rules my characters follow.
The Sith as a religion and an empire are based on the old Sith Empire, which clearly had a very racist caste system. The Kissai led the Empire; they were nobles and the most powerful in the Force. The Massassi were warriors and the most brutal of the Sith races. The Zuguruk built the tombs, droids, and other technology. Then there were slaves, other species as well as some of the lesser Sith who might not have had as strong of a connection to the Force. Whichever caste you were born into, you stayed there. That is what you did because it was your duty to the Empire.
My characters tend to translate that to a nearly hyperbolic conclusion, even to the point of disliking those who aren't red-skinned Pureblood. My characters believe that there is a certain status that every Sith is intended to achieve. Whether that status is predestined or based on skill or talent, a person can not rise higher than that. If a person attempts to overstep that level, then he will be killed by his overlord, but if he steps up and overthrows his masters, then that is the way things should be. But that doesn't mean that a master has to be overthrown. It's possible for a person to sidestep his master and pave his own path.
I base a lot of my thoughts here on the Legacy series of comics, which I think applies to this game because there is one Emperor and a lot of Sith, just as there was during Darth Krayt's time, although I adjusted a few things to fit the timeline. I'd love to hear more of your thoughts about how the Sith religion fits in SWTOR, and I'd love to discuss it in the comments.
The Hyperspace Beacon by Larry Everett is your weekly guide to the vast galaxy of Star Wars: The Old Republic, currently in production by BioWare. If you have comments or suggestions for the column, send a transmission to firstname.lastname@example.org. Now strap yourself in, kid -- we gotta make the jump to hyperspace!