We are using the Beta rules, and one of my players has decided to take the Force Sensitive Exile tree. As a result, my players and I were discussing the Force in Star Wars: Edge of the Empire, and how the Dark Side should work. We decided that Dark Side should become an Obligation; as the player uses the Dark Side it weighs on them, pushing them to ever darker deeds. This is the "rules text" we came up with:
Dark Side Obligation:
Add points of Dark Side Obligation equivalent to the number of Dark Force Points used, even if those points were converted to Light Force Points by spending Destiny.
If your Dark Side Obligation comes up, then during the scenario you will have "opportunity" to react to some thing(s) in the quick, harsh manner consistent with a Dark Side Force user. It will require a Discipline check with a difficulty of one upgraded by one for every 5 points of the obligation, rounded down. Failure means the Dark Side controls the character's action. Success means the player controls the character's action. If the player is successful and chooses actions in keeping with the Light Side, his Dark Side obligation may be decreased at the end of the session.
So first off, I think obligation is a great way to handle the mysterious-bad- stuff-that-you're-gonna-regret that is supposed to happen for characters that draw on the dark side.
That being said, I don't know if the implemenation described here is where it should be. First off, Theres no mention of the ST reduction if your obligation comes up. I think its a perfect representation of the "the cost of the dark side". Second, the compulsion to act in a darkside manner sounds more motivation-y, than obligation-y, but theres no reason it can't be both.
I do like the feeling that each time you pull on the dark side, it clicks your motivation up. It may be wise to put a cap on the amount of obligation you can acrue before being fully corrupted by the darkside. I'd recommend something like Willpower x 5. This mechanic is intended to be similar to the corruption mechanics seen in both the WEG and WotC editions. If the player exceeds this threshold, the GM may declare the character is now an NPC and use him accordingly.
A larger issue is allowing the character to reduce the obligation be successfully making a Discipline roll and making a light side choice. The ability to reduce obligation should be pure player choice, and not reliant on a skill check. Second, reducing obligation, especially dark side obligation, should be a hell of a lot more difficult than what is described above. Atonement to remove 5 pts of Dark side obligation should be enough to be an adventure.
Just some thought on refining how it's implemented. Great foundation idea.