Probably the two most horrifying incidents in Lord of the Flies, by William Golding, are the murders of Simon and Piggy. No one wants to believe children are capable of such brutality, but clearly they are, both in the novel and in real life. The two events I find most frightening, however, are different than this.
The first event is the accidental murder of a littlun; actually, it is the response to the boy's death which I find frightening. The boys run to light a small signal fire, but things are so out-of-control that the fire becomes a conflagration which devours an entire portion of the mountain--as well as one little boy.
“That little ’un that had a mark on his face–where is–he now? I tell you I don’t see him.”
The boys looked at each other fearfully, unbelieving.
“–where is he now?”
Ralph muttered the reply as if in shame. “Perhaps he went back to the, the–” Beneath them, on the unfriendly side of the mountain, the drum-roll [of the fire] continued.
This is an ominous foreshadowing of what is to come, and if the little boy did not have a mulberry birthmark on his face, he would not even have been missed. That is frightening.
The other frightening moment is when Jack paints his face for the first time. Jack has already become obsessed, but it is clear from Golding's description that worse is yet to come.
He looked in astonishment, no longer at himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness. The face of red and white and black swung through the air and jigged toward Bill. Bill started up laughing; then suddenly he fell silent and blundered away through the bushes.
Now that Jack no longer feels the constraints of shame or a conscience, anything is possible--and he knows it. Even worse, one of his own hunters, someone who has known Jack to be a cruel and intimidating leader, sees Jack in the face paint and is frightened because he realizes the implications.
The beauty of a novel like this is that each person will react to it differently, so readers will all be frightened by different things.