Its always the way the characters look at each other, the look of love in eyes that makes one fall in love with the characters. It's same here too, its just that she falls in love with a bad bad guy and you kinda end up falling in love with them. I was stunned, moved and touched by the way their relationship molded over time. The script doesn't offer you with much shocking plot twists but does give slow burn twists that you would have expected for a long time but it still make you feel shocked when you see it. The aspect of finding someone you feel like you belong with though you shouldn't - that drives the movie. The gore killings, yah can weird you out. The slow burn sexual connection and emotions are put out in an excellent way by the actors.
The script and the acting makes it feel so raw, the essence of the movie.