Robert Siodmak's 1946 thriller The Spiral Staircase is a clear precursor to the slasher films of the 1970's and '80's, for anyone who thinks that genre somehow spontaneously erupted in that era. It's got a serial killer who preys on vulnerable women, in this case, those with some kind of disability, and in several scenes, the camera takes the killer's point of view, as he stalks his victims. And as in most slasher films (and really, as in any murder mystery, of which slasher movies are just a gorier sub-genre), the killer is not identified until the very end.
Of course, this being a movie shot in the mid-1940's, it's very tame, but does contain some memorable imagery, (see the above gif!) and a great performance by Ethel Barrymore as the old woman our mute heroine Helen (Dorothy Maguire) must care for. RKO regular Kent Smith is also good as the requisite love interest.
The one thing that puzzled me is why the story was set in 1906, and yet nothing in the storyline, aside from a scene involving a silent movie, requires it be set at that time. Normally I wouldn't really care, but the costuming, as is so often the case with period pieces shot way back when, is just way off, particularly for the men in the film, who look like they'd be able to just walk off the set and hail a cab home.
I watched the film on DVD, but it appears to be available on YouTube, and is embedded below!