Set aside the place of the picture in Truffaut's oeuvre, along with the extent of Hitchcock's influence and the literary antecedents of the story; in other words, take a blindfold test, so to speak, on the movie itself. If created by J. Doe from a story by R. Roe, this film would generate lukewarm praise at best - none for the credibility of the plot or main character. The narrative, presented in a naturalistic, matter-of-fact manner, is in fact completely contrived to serve the purposes of a bizarre story. The leading character, whether to be viewed as suicidal or homicidal (or both), is presented not as psychotic, but as in total emotional control of herself - as self-composed and methodical as she could possibly be.
It just doesn't work. The stories Hitchcock used were developed from simple, seemingly random everyday encounters, the kinds of mishaps or minor aberrations that might touch any of us. Such an approach here might have made the picture more worthy of serious appraisal.