The Beastie symbolism

The "beastie" is a symbol of evil.... the evil that lurks inside of everyone.

The beastie symbolizes human nature and how it is evil. It signifies how it is within themselves and not physically there. The beast is not a real creature, although the boys believe that it is real. It is just their human nature that is pure evil that will eventually unleash itself which will lead to a crime that fully controls a person.