Night Must Fall is widely regarded as the forerunner of a number of stage and screen plays featuring psychopaths. I suppose you could describe Dan, played originally in London by the author himself, as Hannibal Lecter in a bell-boy's pillbox hat. A review of the play in 1935 was headlined "A study is scizophrenia." Later Williams confessed that he had to look up 'schizophrenia' in the dictionary.
Whatever emotional disability afflicts Dan, he is a complex character and his beaviour is the catalyst for what London Times in 1935 described as "a study of vanity, the vanity of the bore, the vanity of old age, the crazy histrionic vanity of the criminal." In the late 1990s Night Must Fall betrays a certain quaintness and tendency toward the "spine-chilling melodrama referred to by an American newspaper in 1936, but it is still a well made play, and is of the accessible kind.