A very creepy minimalist horror movie...
I Bury the Living
(1958)

Richard Boone reluctantly takes over as chairman of a cemetery. A cemetery map has black pins for occupied plots and white pins for reserved plots. Boone places a black pin in the map by accident and the owner of the plot soon dies. After further experimentation (and more deaths) he decides to undo what he has done and replace the black pins with white pins.

He then discovers the graves open and the bodies gone. Just as we are about to prepare ourselves for a supernatural ending we find out that the Scottish caretaker forced into retirement is responsible.

Thanks Don P!