...and, in one of the many instances of irony thoughout the novel, Simon, who is the only character that recognizes the "beast" for what it truly is, is killed as a result of that growing beast while attempting to expose that there is no "physical" beast!
Golding uses "the beast" as a symbol for the savegery that lives in each and every human being. At first, the imaginary beast is an unknown that the boys are afraid of. They are afraid of it because they do not know what it is and they are not familiar with "the beast" living inside each of them.
Simon is the only one of the boys who realizes that the beast is not real, and the only reason that the boys fear it is because it exists in each of them. The rest of the boys, however, become more and more savage. As the boys become more savage, the beast becomes more and more real to them. They fear it intensely and treat it as if it were some sort of god; leaving it sacrifices and offerings to placate its whims.
The boys don't realize that it is their behavior that makes the beast real. They don't realize that the more savagely they act, the more real the beast becomes.
Golding uses the beast to symbolize humanity’s irrational fears and how they affect human conduct.