In 'Pet Sematary', it's the distortion of something as natural as death and the afterlife. We expect death to be final, but when that's subverted and the dead return in a wrong way, it messes with our fundamental beliefs. King is a master at taking normal things and making them into sources of horror. He also has a great way of creating a sense of unease and building suspense throughout the story. For instance, in 'The Shining', the slow descent of the father into madness as they are isolated in the hotel is a gradual build - up of horror that keeps the readers on the edge of their seats.
Well, King's ability to create vivid and disturbing images also contributes to the scariness. In his novels, he describes grotesque scenes and terrifying creatures in great detail. For example, in 'It', the description of Pennywise the clown with its sharp teeth and menacing presence is enough to send shivers down your spine. His characters are often complex and flawed, which makes them more relatable, and thus, when they are in terrifying situations, it becomes even scarier for the readers.