If this belated revelation changes nothing from one perspective — Oswald still did it — it simultaneously changes everything, if only because it disrupts the state of mind of everyone who has ever been transfixed by the Zapruder film. The film, we realize, does not depict an assassination about to commence. It shows one that had already started.
The "transfixing" that goes on when we look at the Zapruder film, is then directly linked to the assassination being a structural part of the film--and, one could say, the structure of any film in general. What is so interesting about documentary films like that of Zapruder's is the fact that they always are viewed like evidence. The film here itself betrays this general quality of film because it was itself used for evidence.
This is similar to what Benjamin has to say about the structure of anything filmed or photographed: photos are photos of a crime. That crime, for Benjamin, is always murder--or assassination. Why? Because photos always document the death of experience, and thus are always already photos of dead people. To be a little more precise: photos and film always capture an experience that is staged for the camera. This doesn't mean that people in films and photos are always posing for the camera--Kennedy in this instance obviously is not--but that reality is becoming filmable and not what the camera is supposedly supposed to capture: "authentic" experience. This experience, then, always lies outside the filmable--in the Zapruder film, experience (and specifically the real experience of the assassination) lies within the gap in the filming. In the end, this is what is appealing to Benjamin about the photos of Atget--as he says in "The Work of Art in the Age of its Technological Reproducibility" they display unabashedly the structure of the photo as the death and murder of experience:
The cult of remembrance of loved ones, absent or dead, offers a last refuge for the cult value of the picture. For the last time the aura emanates from the early photographs in the fleeting expression of a human face. This is what constitutes their melancholy, incomparable beauty. But as man withdraws from the photographic image, the exhibition value for the first time shows its superiority to the ritual value. To have pinpointed this new stage constitutes the incomparable significance of Atget, who, around 1900, took photographs of deserted Paris streets. It has quite justly been said of him that he photographed them like scenes of crime. The scene of a crime, too, is deserted; it is photographed for the purpose of establishing evidence. With Atget, photographs become standard evidence for historical occurrences, and acquire a hidden political significance. They demand a specific kind of approach; free-floating contemplation is not appropriate to them. They stir the viewer; he feels challenged by them in a new way.
One could easily see that Benjamin would have similar things to say about the Zapruder film and how it challenges us, transfixes us. If this all sounds a bit too "theoretical," ponder this: isn't that which is fascinating about the Zapruder film the fact that the camera is only one of the devices that is pointed at Kennedy at this moment, tracking his movements? That is, that what gets viewed (from a different perspective, of course) in the scope of a rifle pointed at Kennedy would look the same way as the film?