These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
A lot of people seem to think Hayley was molested or otherwise abused, driving her to seek revenge on men like the one who hurt her, which changes things. Page even mentions on the blu-ray commentary that Hayley being raped by her father was one of the backstories fans brought up for her, but gave no confirmation.
Haley was just in Jeff's head. The fact that she went to such effort to clean up all evidence that she was ever there, and the importance she placed on him killing himself, could be interpretted as a sign of this.
Moral Event Horizon: Both characters cross it. Jeff crosses it offscreen, in the backstory, by virtue of being a predatory hebephile, Hayley during the castration scene.
Narrowed It Down to the Guy I Recognize: Surprisingly subverted. Sandra Oh, despite appearing in several famous movies and shows (including a starring role in Grey's Anatomy) has been placed in a One-Scene Wonder role as an extra; despite being very suspicious of Hayley, she disappears for the rest of the plot (besides Jeff using her suspicion to try and make Hayley paranoid) acting as a human Red Herring of sorts.
Rooting for the Empire: Though both of the protagonists themselves are established as villains, some fans found it easy for root for Hayley herself.