Where does Simon go after he leaves "the Lord of the Flies," and why do Ralph and Piggy decide to attend Jack's feast?
After Simon's vision of the Lord of the Flies, he falls into an epileptic seizure, and after that he sleeps. When he wakes up, it is almost sunset. He asks himself, "What else is there to do?" and then heads through the wooded part of the island, up to the mountain top, in order to discover for himself what the "beast" that Samneric and later Ralph saw really is. He gets all the way to where he finds the corpse of the dead paratrooper bowing and sighing in the wind. The decaying body is full of flies and causes Simon to throw up when he examines it. He loosens the ropes from the rocks that are holding it; presumably this stops it from bobbing up and down in its reclining-to-sitting motion.
After that Simon descends the mountain. He is determined to let the other boys know that the thing they feared so much was "harmless and horrible." Because of his physically demanding day—his seizure and then his strenuous climb up the mountain—he can only stagger as he walks. He makes his way toward the beach where he can see the boys have gathered.
In the meantime, all the boys have gathered for Jack's feast except Piggy and Ralph. Ralph derides the boys for flocking to Jack's tribe, but Piggy seems tempted by the meat. He talks Ralph into going "to make sure nothing happens." This ends up being highly ironic. Of all the boys, Ralph and Piggy are the ones who are thinking most clearly and should have been able to make sure nothing happened—but they end up being part of what happens. At the feast, Ralph and Piggy eat their share of meat, but they stay on the outside of the group. After Ralph has a verbal confrontation with Jack, Piggy tries to get him to leave, saying, "We've had our meat." The lightning lights up the sky, and Ralph may have been able to lure the boys back to his side by the thought of the shelters, but Jack entices them all into a dance. Even Ralph and Piggy join in because they "found themselves eager to take a place in this demented but partly secure society." That is, their desire to be protected from the storm by being part of a larger group keeps them from leaving. That is how they end up becoming part of the frenzied mob that murders Simon as he crawls onto the beach.
After Simon leaves his secret place where the hunters have put the pig's head, he staggers down to the beach. Unfortunately for him, the boys are in a complete frenzy, doing their hunting dance. Even Ralph has joined them. This is because he is a bit afraid of the storm that is blowing up. Jack says that they need to dance to prevent the storm from hurting them.
When Simon gets down to the beach, the boys end up treating him as the pig. They beat him to death. They do this because they think he is the beast.