...they telegraphed who the bad guy was. I mean, the instant the person appeared on screen, I knew it was the killer/stalker. That ruined any sense of suspense the movie could have had. So there was nothing else to keep my interest after that. It was especially bad since it happened relatively early in the film.
The technique of always hiding the bad guys' face throughout the movie just affirmed who it was. That was just very poor writing.
Another reviewer here said "The filmmakers kept the audience guessing about his identity". meaning the identity of the bad guy. Are you kidding me? Anyone who didn't know who the bad guy was as soon as they appeared should be ashamed of themselves. Judging by the number of high ratings in some of the reviews here, lots of people share that shame.