It comes down to a lot of different factors. The Jedi are generally shown as the heroes, while the Sith are shown as the villains; then again, running around hewing off limbs isn't exactly a nice thing to do, and the Jedi are rarely shown in their diplomatic and peace-making roles because it's not exiting and doesn't make for a good game/movie. The same could be said for Sith as well.
What it comes down to is that the Jedi try to sacrifice everything of themselves for everyone else, while the fallen Jedi who became the Sith try to sacrifice everything else for themselves, which has been the general theme of Star Wars. Can you have Sith motivated by compassion, hope, or love, as well as or instead of anger, fear, or hate? I don't see why not, any less than you can have a detached, methodical Jedi reason that sacrificing a few lives for the greater good is entirely reasonable. While the archetypes aren't presented as such, it's certainly possible. Each order doesn't support that character, who would normally be brought to heel or destroyed in normal times, but the possibility exists.
Now, there's the difference between Sith Lords and Jedi's overall roles; Sith are rulers, and while I agree slavery is wrong, so is massive bureaucratic corruption and self indulgence. Considering the state of Coruscant when the game starts on the Republic side, you can find many deep issues within the Republic's government as well. Is a system that condones slavery for their weakest members but gives them rights and social mobility when they've demonstrated they can take care of themselves be better than a self-interested and corrupt democracy that caters only to the powerful and leaves it's citizens trapped and helpless? While that's not the default setting for Star Wars, it shows how just because we're more comfortable with the inequities of one system doesn't mean that it's automatically better than another system.