if Jaesa got darksided (she gets insta goth makeup), doesn't matter what you do afterwards. It's the same with the other companions, like Vette. Vette will certainly dislike a sociopath murdering bastard uber d-bag, but just spam enough gifts and you can marry her.
Okay, now how DO I make her dark side?
You know for sure? Cause I'd like for my Sith Pureblood that is going to romance her to not be forced to be all dark and kill everyone just for kicks unless it's necessary. Trying to play him as a loyal Sith, yet not a sadistic jerk.