One major flaw. (Spoilers of course)
I don't care about the "She should have done this or that" scenarios since not everyone thinks or behaves the same sitting in the comfort of their home, than if they were put in a real situation like this. What I do care about is how the killer got in the back of her in the bathtub when the only access in, was the door, that she was looking straight at, or the side window. Since she was staring at the door the entire time, then we know it wasn't that way. And the side window can't work either because she could see anyone entering it, in her peripheral vision since her sight and feel are the only things she got going for her. Most definitely, the guy's body would have obstructed the moonlight and created shadows that would be brought to her attention. Now the only thing I can think of is either she dozed off for a few seconds which it didn't show as her eyelids at most, opened and closed slowly, and I don't like to assume here, or that her focus was off except for on that door because of her fatigue and blood loss which was indicated with her dreamy like gaze. Any thoughts?
Aside from that one scene, I thought it was pretty good. Realistic and nothing done there that couldn't happen in the real world.