Simon is delirious and half-dead, both from his seizure experience with the Lord of the Flies and the attack by the boys who thought he was the beast.
At the end of chapter 8, Simon sees the Lord of the Flies and learns the true nature of the beast, that it is the boys themselves. He speaks to the beast, which is really an internal conversation or hallucination. He then submits to the beast. As the chapter ends, he is having a seizure.
Simon found he was looking into a vast mouth. There was blackness within, a blackness that spread. (ch 8)
Simon is not like the other boys, but just because of the fainting but because he is reflective and introspective. When he goes off into the woods, he further distances himself from the boys. Since he is the only one that knows what they truly are, metaphorically, symbolically they must destroy him.
This is why, in chapter 9, Simon is incapable physically or emotionally of defending himself.
Simon spoke aloud to the clearing.
“What else is there to do?” (ch 9)\
Since there is nothing to do, and the beast is “harmless and horrible,” Simon decides that “the news must reach the others as soon as possible.” He is physically weak and can barely make it. When he arrives, the boys are deep in their beast-dance trance and barely recognize Simon “crying out something about a dead man on a hill” (ch 9). They take him for the beast and attack him.
The attack on Simon symbolically acknowledges that the boys are not ready to recognize their true nature and the dark force inside their selves. Instead, they continue the violence and Simon is the first casualty.