He claims to see a monster, a snake, or beast at night.
He is never seen again. It is presumed that he dies in the massive fire at the end of chapter 2. This gives gravity to the idea of a monster because there is no proof of his disappearance.
Roger throws rocks at Piggy on the beach.
Later, Roger rolls a large boulder down a cliff, crushing and killing Piggy.
Simon
Simon talks to the 'lord of the flies', the embodiment of the monster all the boys fear. He tells Simon he is going to have some fun with him.
Scared Simon runs out of the woods and the boys think he is the monster and attack him.