The use of shadow emphasizes the fear of the unknown we as an audience experience when watching a horror film. The mind will create a scenario different to each individual, as we know what scares us the most and will play on it when the film does not show us certain aspects of itself. Shadows often exaggerate the object/person's form they have taken, such as count Orlok's deformed appearance in "Nosferatu", and will also change the atmosphere and mood of a scene negatively.
A modern example of the use of shadows for these reasons is "Jennifer's Body", candles are used to cause the shadow to flicker slightly and as they often represent romance, they contrast with the sillhouettes of Jennifer and her first victim as well as the actions in the scene; the violent killing.
