Theories

Verhoeven claims that the movie is more about the way Reve perceives the world around him and imposes things upon the reality/realities in which he finds himself, but the film allows you to interpret it in a much more literal way, if you wish to--much like BASIC INSTINCT never fully establishes whether Beth, Catherine, Roxy or a combination of all three of those characters is the murderess. For me, the deliberate ambiguity makes the film a richer experience.

'What does it matter what you say about people?'
Touch of Evil (Orson Welles, 1958).

I don't know (nor does anyone) if Gerard was just imagining the whole thing or if Christine really was the supernatural murderess, however, even after watching it the first time having no precursory knowledge on the movie, i found it ambiguous and slightly leaning to the idea that Gerard was a bit of looney. There is nothing to suggest that Christine murdered anyone other than her semi-suspicious manner and alluring/seductive character. Obviously her motives and mindset are never made clear, but what is made clear from the beginning is Gerard's indulgence in his imagination, and one questions whether his 'visions' are merely his obsession with death/religion/etc. From the beginning he states at the conference that he 'lies the truth,' and while it is clear that there is an abnormal amount of coincidences regarding the 'Mary' figure and the visions of the future, I think the audience is ultimately led to question Gerard's judgement and his perverse visions (or imagination?).



Who ever heard of a fatalistic farmer?

I've just watched this in a film festival (thank God there still are 35mm projectors), and I think the film leaves two different trains of thought to the audience: either Reve's visions are true and she is some kind of black widow witch, or Reve is crazy. The film chooses no clear path, providing elements to sustain both theories.
However, there's something I've been thinking about and I hadn't heard mention of this particular detail as yet. At the end of the film, the doctor clearly states that Reve has a "mild alcoholism", noticing a slightly oversized liver. Still, I know for a fact that SEVERE alcoholism may cause hallucinations and erratic behaviour, both on account of the drug and of consequences of cirrhosis (A friend's father of mine has recently died with cirrhosis and he displayed this behaviour, even though he wasn't drinking anymore).
That would partially explain the first scene, in which Reve is clearly having abstinence effects of alcohol addiction and ends up killing his partner back in Amsterdam -- I take it this scene takes place after the events depicted in the film and that he's gone totally insane.