The two, in both the novel and the play, thematically represent the destructive forces within man. In Macbeth, the three witches are the personification of evil and in Lord of the Flies, the beast is an imaginary evil creature existing within the boys' consciousness.
In Macbeth, evil is represented in its physical form by the three witches and their leader, Hecate. The witches' strange appearance and disappearance on the heath when they encounter Macbeth and Banquo, epitomize their supernatural design. They are abnormal creatures and come to represent, to Macbeth, his destiny. He becomes dependent on their predictions and is a willing victim of their manipulation.
The witches tap into Macbeth's innate evil and exploit it to fulfill their pernicious purpose which is to overturn the natural order of things and encourage evil. It is they, as the agents of the lord of all malice, Satan, who motivate the gullible Macbeth and drive him to commit the most pernicious acts: the betrayal and murder of his king and the assassination of his confidante and friend, Banquo, as well as the nefarious annihilation of Macduff's entire family. It is, however, Macbeth's innate evil that truly drives him and once he has awoken his blood lust, it overwhelms and controls him.
In Lord of the Flies evil is represented in physical form by the dead parachuter which Sam and Eric see and assume is the beast. The 'Lord of the Flies' which is a grotesque and decaying pig's head stuck on a stick, becomes to Simon, the personification of evil with which he has a philosophical discourse during a hallucination. He believes that the pig's head is talking to him and he concludes that evil, the beast, exists within man. Evil and all its elements are innately part of man and cannot be avoided.
Simon, in a hallucinatory state achieves a moment of profound enlightenment and sees that evil is not an external, separate force, but that it is an innate and inseparable part of us. It is, essentially what makes us, paradoxically, both man and beast at the same time. We, therefore, choose to be civilized or not, as Ralph and Jack clearly display. We decide into which force we tap into and what drives us.
Simon also discovers that what Sam and Eric had determined was the beast, was in fact a dead parachuter and, ironically, his attempt at informing the boys about this results in his murder. He wanted to save the boys from their fears but, in the end, he becomes the target of their dread because they believe that he is the beast and brutally kill him.
It is obvious that evil, as it is represented in both genres, is an innate, dormant force within all men. Once the beast within us is awakened, its path of destruction can go unchecked and can only be stopped or contained by a more powerful force, either internal or external. In Macbeth, that force is represented by Macduff and his allies, whilst in Lord of the Flies, it is epitomized by a naval officer.