There’s also the possibility that Rey was really angry and pissed off at Ren by that point, and was dipping her toe in the Dark Side, much like Luke did when he beat Vader in ROTJ. All the more reason she needs Luke to teach her now.
In the novelization she even hears a voice that’s implied to be Snoke’s telling her to finish Kylo off while he’s down. I think she’s definitely tapping into something not entirely wholesome in that scene.