Fiona archetype (Ninth Gate)
For those who have watched The Ninth Gate we have "The Girl" a supernatural being that works as a guide for the chosen one (Dean Corso) to realize and accept his destiny and follow the dark path that leads to equalization with the final cause, in Kill List, Fiona descends with Jay in his inner depravity to manifest it explicitly and to make him embrace it. The selection was depicted early in the movie when she marks their mirror just like "The Girl" marked Corso's forehead with blood.
There are even implications of Fiona being a supernatural entity again with an analogy from Ninth Gate (sex scene near the end, with the demonic eyes), Gal describing her as a demon during their sexual intercourse. We also get the scene outside of their hotel room where Fiona waves at Jay as an ethereal projection with the white dress late at night. That scene might also be a representation of Jay's mental instability as he descends to insanity and gets detached from reality. Or maybe it actually takes place in the real world and she is a crazy bitch herself.
There are more clues and hints throughout the movie but i made the mistake to watch it when i was extremely tired late at night and without turning the subtitles on, having the false impression it was going to be one of those generic action/hitmen type of movies, i am unsure if it was an audio issue or their accents, but i was unable to fully comprehend their dialogues during the first act (dinner table); being a foreigner i never had similar issues with American accents but with some Australian and British flicks it just feels safer to have the subs enabled. I intend to re watch it as i am sure i will be able to appreciate it even more after a second viewing.