Well obviously it's a question for the next one where she'll explore her own command of the force during her training with Luke, and as we observe more actions. The claim that was made is the question of her struggling with the dark side wasn't even opened. When you recognise she's fighting aggressively, swinging savagely, and feeding off her emotion, traits which have all been portrayed as traits of one whose fighting is empowered by the Dark Side, how isn't that opening up the question of her having a struggle with the Dark Side? The more contrived thing to happen in that fight is for her, while fighting aggressively and feeing off her emotion, walking to Ren with a clear motive of killing him, is to suddenly stop and consider "Wait, I'm considering killing him, I may fall to the dark side" when she has no memory of guidance on the force and how one falls down that path. The fight and her style of fighting is reminiscent of Luke and Vader in Return of the Jedi, and the only reason Luke ended up sparing him, and had a moment of introspection, was due to the goading of the Emperor. Even as a user above mentioned, the cliffs separating was contrived because if it didn't we wouldn't have two more movies, because Rey would have killed Ren then and there. It opens the idea of her struggling with a dark side, which is fine, it's the first movie of a trilogy, she's not going to work through it all while her friend is lying there dying, the planet is collapsing, and Ren is lying there wondering what on earth she's doing standing there talking to herself deciding what to do with him.