Like with most great films each frame tells the whole story. Just look at how terrifying these images are, even without know the story, they are chilling:
The expressionistic lighting and cinematography (done by Stanley Cortez) builds the unsettling mood. Sound is also a big part of this film, in the same way that the music in Jaws, lets the audience know that something is off, the Rev. Powell's singing acts as a warning that things are going to get ugly. Watching this horrible, creepy man terrorize these children, chasing them into attics, and pursuing them down the Ohio river, is more suspenseful and terrifying than any slasher film. This is one of those films that really gets under your skin, and makes you feel ill. It depends on all of the elements of film to bring out the horror, something I wish more films would do today.
Here's a clip that shows what I mean. The extreme dramatic lighting in this scene really makes it. It heightens the mood and the suspense, and without it the scene would not be nearly as successful.