I'm suprised this is really in issue...
Actually... she was both neutral and darkside ('evil'). She starts the game neutral aligned and then later returns to her dark side alignment when she feel she has reached a conclusion about the 'exile' and leaves the party. Just check out her alignment anytime she is in your party and you will clearly see she is neither lightside or darkside... she is neutral. If you have read some of the expanded star wars universe lore you will notice that, occasionally, characters who are Sith or Jedi strive to remove themselves from their the stereotypes associated with either 'side'. In Kreia's case she is fascinated, absorbed, and even obsessed with an anomaly in the force... not unlike a scientist so captivated by an experiment that they don't care if the results will ultimately prove them right/wrong or affect them personally. This is not unlike modern politics where individuals of a particular affiliation (republican/democrat) diverge from their own parties take on an issue and become so focused on the principles of the issue itself that they rise above the labels that have been placed on them.
Darth Sion makes it clear that Kreia aka Darth Traya (a Sith Lord and thus evil) was obsessed with the exile because he/she was able to walk the line between Sith and Jedi, exiled not just from the order, but from the Force. Early on you learn that she too is an exile from the Jedi Order, and like every other Jedi who left the order, became a Sith. She wanted to observe and learn from the exile and share in the exile's experience of being 'divorced' from the force and returning to the force all the while walking the line between Jedi and Sith. She was testing the exile becuase she/he was a 'wound', an anomaly, in the force and could see that the exile wielded the force still, albeit subtly, in the way the exile drew other force sensitives to him/her despite supposedly being separated or exile from it. She wasn't on a power quest in the tradional sense. She was... 'enamored' with the force power the exile wielded while in his/her exiled state, neither Jedi or Sith. Which is why she seems more pleased when you don't take action to either good or bad extremes in the game. Once you lean toward the light or dark then you become just another 'fallen Jedi' or 'redeemed Sith' ...like all the rest. You can really see this regarding the Jedi. If you kill the Jedi she is displeased. If you don't then she kills them anyways. It's not that she cares whether the Jedi live or die... she is only fixated on your actions and what they say about you. She knows she's an evil Sith, she's only interested in knowing where your power to let go of the force comes from and any suggestion you make by your actions or speech that your power comes from being Jedi or Sith, only lowers your value in her eyes.
Ultimately there is no way to win favor of Kreia simply because a) she 'cares' & protects you only in the same way a scientist regards a lab rat and b) you eventually have to take action in the game that displeases her one way or the other to some degree.
