Pretty much all Sith philosophy is built around the idea of gaining personal power at the expense of everything else by any means necessary. That sort of thing doesn't tend to attract very nice people, and anyone who did lean more towards being a nice guy would get killed by the assholes.
That is to say nothing of the very black and white, good vs evil, light vs dark nature of Star Wars as a whole. If you wanted to rationalize it somewhat, consider that the both the Light and Dark sides of the force sometimes seem to be presented as ethereal entities each with motives and goals of their own.