We have always found horror entertaining. Many classic children's stories are full of horror. This is definitely true of the tales put together by the Brothers Grimm.
Horror stories have been around for a very long time. There is a debate whether Little Red Riding Hood began in France or Germany. Either way, it is an early European werewolf horror story aimed at teaching children not to trust strangers. There is the wolf, the little girl and granny. A woodsman comes to the rescue. The wolf meets with a violent end.
Even the Everyman play that possibly has its origins in Germany and became popular in Medieval England has its elements of horror. Death calls but who will travel with Everyman on the final journey?
If horror didn 't have something useful going for it then how could it have survived for so long as a genre in our collective and rather universal fiction? Stories of wicked witches can be found in Asian countries such as Japan as well as in the west. Ghost yarns are not limited to any one country or time.
Writers as well as film makers have used the format of horror to question modern technology and where we are going with it. The novel Make Room! Make Room! and the movie which followed, Soylent Green, questioned increases in population caused by people living longer but the birth rate not decreasing. In Soylent Green an apple becomes a precious object and most people, whether they know it or not, are actually eating deceased humans. Both novel and book may be classified as science fiction but science fiction has often been said to be horror set in some future time or with new technology creating problems.
Possibly the first book to challenge science and to question where we are going with it was Frankenstein by Mary Shelley. A man creates a new being then deserts it out of fear and revulsion. Who then is responsible for the deaths that ensue?
Horror is books and in movies has always been society's safety valve. It releases our tensions. You think you have it bad in your life at least you are not being chased down the road by man eating zombies. There is a certain element of unreality to the old monsters such as the walking cadavers, zombies, werewolves and vampires where it is a struggle for the writer or film maker to make them appear real enough for suspension of disbelief. Even so, I do enjoy the occasional horror movie. And the harm in them is having them cut up and/or banned.