I have been thinking about this for a while we assume she is a Jedi but in her fight with Kylo Ren she was getting angry. Even after she had that Obi One Kenobi moment she was still getting angry while she fought Kylo. She also has no memories said for some nightmares but Kylo is around her age so assuming Luke's order fell when they both were kids it is impossible to think Kylo pulled an Annakin. However I have a theory I think Rey was swayed by the dark side. She was naturally gifted but ruled by her emotions. Thus the bad guys got to her I think Kylo tried to stop her but failed thus causing him to seek more power as a way of compensating for his failure. Rey being a child was likely sparred by Luke who probably wiped her memories and left her on a desert planet hopping she would live out her life as normal. I think this because Han said that Luke was supposedly betrayed by an apprentice no one not even Kylo actually said it was Kylo Ren. It will probably be an Empire Strikes Back style twist with Kylo saying Rey "I didn't betray Luke you did."
I don't think a person is naturally one way or another. Being a Sith is a choice, like deciding to rob a bank or be a murderer. Rey is an inexperienced Force user. She's just testing out her abilities and has never been taught a right or wrong way to do anything. Even if she had given in to anger as a young child--every kid has temper tantrums, and that doesn't make them a Sith or even a Sith in the making.
Tempted by the dark side in that fight? Yes. Sith? No. The main point is literally anyone no matter how 'good' is capable of being at least tempted by the Dark Side. But, just as Luke did, she will overcome it.
Actually Sith are fallen Jedi. So you don't need a Sith to be a Sith. Also I think Kylo Ren qualifies as a Sith.
“Kylo Ren is not a Sith. He works under Supreme Leader Snoke, who is a powerful figure on the Dark Side of the Force.” JJ Abrams EDIT: I'm a Sith. You're a Sith!
Some Sith were fallen Jedi, but they follow Sith teachings and philosophy, so without that, a fallen Jedi is just a darksider.
Nor was Maul. And no, I don't think that she's a Sith. Being angry/passionate doesn't make you a Sith, it makes you a person. Luke got angry in the OT, to the point of beating down Vader in a fit of blind rage. Obi Wan got angry in the PT, yet neither of them became a Sith. Tempted by the Dark Side and needing to struggle against it, sure. But that's a very different thing.
I don't think shes a sith like this theory proposes, I think she will experiment with darkness a lot more than Luke did though. And turn.
It would be a challenge to the scriptwriters to seriously tempt her to go Dark. It doesn't have to succeed. It just has to be subtle enough writing to show us that it is possible. The Temptations of Anakin and Luke were the weaknesses of both trilogies. Just didn't convince. I just watched Jedi again.... if the Emperor would just STFU, Luke would get really angry and kill his Dad. But he'd open up his poor dental hygiene "Good, good *cackle*", and snap Luke out of it.
There's a good chance Rey has dark side origins. Who's to say she wasn't born and bred to be the second coming of the Sith, but her parents got cold feet at the last minute, and abandoned her on Jakku rather than see that fate befall her? Snoke has to have some personal connection to her, so maybe she was part of some scheme he was working on, but her parents intervened and prevented it from happening.
The temptation to go to the dark side is always present. Anger, rage, revenge are all emotions that lead to it. We've seen every hero tap into it in some way through out the saga. The difference is not giving in to it but controlling it and not letting it control you!
I think it would be so interesting story wise for her to turn, they almost have it set up perfectly not knowing where she came from, real abandonment, she mirrors Anakin in a lot of ways I don't want to see her kill younglings but I would like a scene where she goes to Snoke to learn about the dark side. And turn for maybe half a movie and turn back this would be new we wouldnt have seen it in any of the trilogies. We've seen totally good and totally bad, Luke struggle was not that long for me but it fit the character Rey exploring darkness more, would fit her character. And if it doesnt happen in the films someone please write a fanfic!
I think it is possible but unlikely. A flirtation with the temptation of the power of the darkside might be likely though.
Rey would make a great evil character. It also would never happen. Ever. Disney would never let it happen. Too much $$$ on her being the heroine. Very similar to how Lucas kept Han alive because he was selling too many toys. Regrettably the success of SW has also been it's worst enemy when it comes to some great story ideas.