The mythic horror movie, like the sick joke, has a dirty job to do.It deliberately appeals to all that is worst in us. It is morbidity un-chained, our most base instincts let free, our nastiest fantasies real-ized ... and it all happens, fittingly enough, in the dark.
The author says that those who enjoy horror do so because its a way to let their true self out. And that those who do not enjoy horror are "good liberals." The author also enjoys horror, and watches it to keep the bad hidden, so that only the good shows.