Simon's death comes at the hands of the other boys, who think they are killing a beast. Of course, the Lord of the Flies is the beast, personified in Simon's mind: it is the "darkness in man's heart". So Simon is arguing with the beast, which, figuratively, is going to be the cause of his death. Of course, you could also argue that it is the fear of the beast he is arguing with - which is the more precise way of explaining why the boys end up killing him.
The beast threatens Simon, who is about to reveal the truth of its non-existence:
“I’m warning you. I’m going to get angry. D’you see? You’re not wanted. Understand? We are going to have fun on this island. Understand?
We are going to have fun on this island! So don’t try it on, my
poor misguided boy, or else—”
Simon found he was looking into a vast mouth. There was blackness within, a blackness that spread.
The blackness within is the "darkness of man's heart", it is the evil within all humans. Simon feels it spreading, and feel himself unable to gain control (he is epileptic, and having one of his turns) - symbolically - he is going to be beaten by the spread of the darkness.
“—Or else,” said the Lord of the Flies, “we shall do you? See? Jack and Roger and Maurice and Robert and Bill and Piggy and Ralph. Do you. See?”
Simon was inside the mouth. He fell down and lost consciousness.
And these last sentences are exactly what happens. The boys do do Simon. He is killed. ANd he does fall victim to the beast, and lose consciousness - only that time, for good.