One central message of Lord of the Flies is that humans are savage and barbaric by nature.

When the boys first arrive on the island, they attempt to superimpose the structure and organization of their former lives onto their new environment. They seek a leader, create some basic rules, and recognize the need for cooperation in order to survive.

Yet as time goes by, they stray from their initial desire to live civilized lives. Jack, in particular, leads a quest to usurp power and entices the other boys to join him with promises of blood, violence, and action. His tactics are successful, and eventually only Ralph, Piggy, and Simon steadfastly cling to the ideals of civilized living.

When Simon experiences a "conversation" with the Lord of the Flies, it assures him that the Beast is not "something you could hunt and kill." Simon recognizes this truth early in the book, suggesting that the beast which the boys seek is "only us," or evil that lives within each of them. This sense of wickedness continues to spread throughout the group until eventually they kill Simon; even Ralph and Piggy see Simon as the "beast" in that moment and fail to intervene on his behalf. Life on the island crumbles quickly after Simon's death. Jack's group later kills Piggy and is desperately trying to kill Ralph with the island ablaze around them as rescuers finally arrive.

Without structure and laws, the boys fall into barbaric behavior. It's important to recognize that the oldest boys are only about twelve, and many are much younger. Since children are often associated with innocence, their quick decline into savagery furthers the message that the natural state of mankind is violent and self-serving.