Explain the ending!!! Did she leave him? (Spoilers Ahead)

I took it to mean she got scared and left but returned wanting to give them another chance. I think she stayed back observing him half to see his reaction and the other half because she's returned hoping he hadn't come out to see her gone yet. Her expression didn't seem deviant just startled/scared.

I think that people who engage in those kinds of dysfunctional relationships often want to put their fears and insecurities at bay. One counter-intuitive way of doing that is running from the person you have a connection with and seeing if they chase you, and if so, how far are they willing to go, how much they are willing to suffer in order to be with you. People can get caught in a loop in which if the person stays they feel reassured, but as soon as the anxiety manifests itself again, they start the whole thing over, possibly with even more intolerable behavior. "The more this person can forgive me, the more I'm important / I can be sure he will stick around".
I think she was checking his reaction to her departure because she's so damaged and can't accept, on an emotional level, that someone might care for her. What happens next is unknown, but the usual scenario is that people who engage in these relationships do this whole thing over and over again until the relationship becomes a barren land.