What part seem the most important in explaining what the Beast actually is?

The most important part in explaining what the Beast actually is lies in its representation of human fears and inner darkness. It symbolizes the primal instincts and savagery that can emerge when civilization's constraints are stripped away. Additionally, the Beast serves as a reflection of the boys' descent into chaos and moral decline, illustrating that the true monster resides within themselves rather than an external force. This duality highlights the complex nature of humanity and the struggle between order and savagery.