Ralph, Piggy, and Simon are all allegorical characters. They represent something to the meaning of the story beyond themselves, and when they are targeted and killed, one by one, it is essentially as if these human traits are murdered as well, leaving the boys in complete and utter darkness as far a civilization goes.
Ralph represents reason and logic. He is the original leader, chosen by the boys as a whole. He is the one who orchestrates the collecting of water, the building of shelters, the collecting of food, and the watching of the signal fire.
Piggy, his friend, represent intelligence and insight. Piggy is the advisor of Ralph, the one who keeps him on track and helps him prioritize what is important until the rescue. HIs glasses are also a symbol of that insight. He sees well with them, but is blinded without them. With them, the boys are able to create fire which is the source of heat and warmth, and gives them the ability to cook food for health. When Piggy is murdered, that insight is gone. Ralph no longer has that intelligence to lean upon, and the glasses have become the spoils of war just as the meat from the pigs they've hunted seals their victory over the island wildlife. "Piggy" has become just another member of the hunted wild pigs on the island.
Simon was the first to be killed. He represented innocence. The boys' innocence disappears first as they battle the unknown of the island and of life itself with no adult supervision. The outbreak of war, the beast on the island and the death of Simon, which they all took part in as part of a frenzied attack on something fearful which they perceived as a threat, has forced them to grow beyond the stage of innocence they had before they were shipped off to safety.
Hope this helps!