What is the "sin" nature or the "beast" within the boys?

One of the main themes Golding examines throughout the novel Lord of the Flies concerns the inherent wickedness of mankind.
Golding believed that humans were inherently evil, malevolent beings that would revert back to their primitive state if they were put in a situation without the laws and regulations of society. Throughout the novel, the boys gradually descend into savagery after failing to create a civil society.
While the boys believe that there is a dangerous creature on the island, Simon is the only person who fully understands the true nature of the beast. During a conversation with the Lord of the Flies, Simon hallucinates as the severed pig's head explains to him that the beast is inside each boy and is not something that anyone can kill. Golding believes that "mankind's essential illness" is humanity's inherent wickedness, which each person possesses and is therefore the reason the boys become brutal savages. Golding argues that humans are inherently evil beings and society's laws and regulations are the only forces preventing people from acting like savages.