Who said anything about the Dark Side?
While I know that the films are chucking out the EU, it's worth remembering that the previous films already established that the core difference between the Jedi and SIth's philosophies is that the Jedi deny and repress their emotions, while the Sith actively embrace them? A Sith who chose to embrace positive emotions, wouldn't necessarily be evil or find themselves corrupted?
I could see someone who uses Dark Side powers and is a light sided character, as we've seen people like Plo-Koon use Force Lightning before in the name of good, but I genuinely believe it's impossible to have a light side Sith. There is so much more to being Sith or Jedi than just one 'represses their emotions' and the other 'sets them free'. Being Sith is a way of life, it's a religion. It's something someone becomes because they've embraced the dark side fully and they desire nothing more than to become more powerful. They're power hungry, they're selfish, and they do not like to share to the point that they'll kill off whoever else has it. These are not traits fit for someone who is light side.
To take a few quotes from Wookiepedia:
"The Sith are people who are very self-centered and selfish. There used to be many Sith, but because they hunger for power, they killed each other off, so now there are only two - a master and an apprentice. Sith rely on their passion to get things done. They use their raw emotion, their hatred, their anger, their bitterness - which is the dark side of the Force. The Force is what binds the galaxy together, and it has a good side and a bad side. The Sith learned how to manipulate both sides of the Force, and then they fell into the trap of being corrupted by the dark side. The Jedi Knights are like marshals in the Wild West. It's their job to make sure everyone is protected, to bring peace. They are the enemy of the Sith, because the Sith want to dominate the galaxy, to control everything, and for a thousand years they have had a plot against the Jedi. So, in this movie, it is time for them to seek revenge against the Jedi for perceived injustices and to carry out that plot." ―George Lucas on the Sith
"We take what we desire because we can.
"We can because we have power.
"We have power because we are Sith." ―Sith proverb