I had heard about this movie for years in the same way you hear about The Town that Dreaded Sundown, this was a proto-classic that was way ahead of its time.
The story follows Mark, a young photographer whose father subjected him to very invasive filming techniques growing up in the name of psychology. Mark now hunts women and murders them while filming their final looks of terror. One of the central mysteries of the movie is just what they are seeing in those final moments that makes them so scared. Through the movie, Mark almost develops a normal relationship with a female neighbor despite her mother's feelings that Mark is a creep. In the meantime, Mark just keeps killing any willing floozy he can lure in with offers of free photos.
For one thing, this could have been a found footage movie. Mark keeps his camera running and on him at all times. Often, we get POV shots of Mark stalking his victims but most of the movie is in regular wide shots. The POV gimmick would be used over and over again in movies ranging from Psycho to Friday the 13th. The genius of Michael Powell (the director here) is that he was the first creator to realize the thrill/discomfort of placing an audience in the killer's position. We horror fans are guilty of the same voyeurism as Mark, he just takes it further.
Even though this feels like a trashy Roger Corman movie, it has a lot to recommend it. I believe it made the Criterion collection so you cinema snobs at least need to check it out.