Freud's Concept of The Return of the Repressed

The return of the repressed is where repressed elements from a persons past/childhood is brought from the unconscious to the conscious mind. He believes that we have no memories of events that happen in our early life because of the repression that effects them. The repressed is not destroyed in the unconscious mind, it emerges in the form of a derivative of the unconscious but they only see distorted images as it it still retained in the unconscious mind.
This links to horror as the killer is driven with insanity and kills people linked to what happens in her/his past. This is because their repression is brought to the conscious mind and this causes them to act out not knowing that they are reliving their past experiences. Most film makers play on this theory by introducing the film, like Halloween, into the killers past then showing present day and as we go through the movie it explains more by showing 'flash backs' of the parts that is left in the unconscious mind.