[member="Darth Naomi"]
Fair enough, although I suppose it's more than that. True, the Sith are inherently "evil" in the canon, but largely because it's viewed through the protagonists' point of view, although there's also the fact that the Sith ideology typically promotes selfish behavior, which is usually perceived as ambiguous at the best of times. A few exceptions existed though, most notably Darth Vader (who genuinely believed that through his actions he was doing what was best for the Galaxy by enforcing order).
This allows me to better explain this to [member="sabrina"] :
Basically, most Sith are driven by self-interest, and the problem with giving that kind of power to any psychopath who has the potential is that, inevitably, they'll use it for destructive purposes. The idea of personal freedom among force users championed by General Xendor when he founded the Legions of Lettow, forerunner to the Sith and its various spinoffs, was ultimately a bit too optimistic and naïve, and didn't take into account the fact just because one can have that kind of power doesn't mean that they should.
This is why there's a single Dark Lord as head of the order: to focus the order's efforts into something more constructive for the greater good of the Order and its adherents.