It explains that in the hearts of every human being roams a darkness that even they may not recognize. This darkness includes greed, fear, rage, crushing sorrow, maleficence and overall the manifestations and capability of human evil. This darkness tends to be covert and repressed in the average human being, but once that person finds themselves in a situation that is confusing and frightening it becomes overt and apparent to the unsuspecting person.

It's a simple and well known theme really.