Macbeth's soliloquy outlines the slipperiness of the ladder of power: and outlines his fear that, from the top, the only way is down. First, Macbeth bemoans his lack of security and admits that he is terrified of Banquo.
To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd.
The goodness ('royalty') in Banquo's demeanour ('nature') means that Macbeth is immediately scared of him as a threat to his crown: either because the goodness is genuine, and thereby could prove popular, or because Banquo looks like an 'innocent flower', and therefore must be the serpent under it.
And, of course, Shakespeare puts this terrified doubt into the audience's mind: Banquo's soliloquy at the start of the speech could be read either as an innocent, terrified wondering ('Will these witches affect me too?') or potentially the beginnings (of course, he is murdered before he has chance to DO anything) of a campaign against Macbeth. Fears stick deep - no-one trusts anyone - this is the paranoia of power.
Another key way power is shown in the scene is the section which follows with the murderers: the smooth, persuasive way Macbeth manages to convince the two murderers that Banquo was their enemy and should be murdered. Macbeth may be paranoid - but, with a single conversation, can bring about the secret murder of a high-ranking official. And why? Because Banquo might be a threat. Power begets fear.
ActIII Sc.1 contains two soliloquies which provide interesting insights into the minds of people who are ambitious and power crazy.
The scene opens with Banquo's soliloquy which reveals his cunning nature:"May they not be my oracles as well/And set me up in hope?" If Banquo had been a truly patriotic citizen of Scotland he should have revealed the prophesies of the witches to the friends of Duncan the murdered king and joined hands with them and should have killed the villain Macbeth. On the contrary, he reasons within himself that since the witches' prophesies have come true in Macbeth's life they will prove true in his life also, "myself should be the root and father/ Of many kings;" and in his over eagerness to see this prophecy fulfilled in his own life, he turns a blind eye to the villainy of Macbeth. It is only poetic justice that his cunning nature results in his own death at the hands of Macbeth.
Macbeth's soliloquy in ActIII Sc1 reveals to us the evil workings of his power crazy mind. Not satisfied with having his ambition of becoming the king of Scotland being fulfilled by evil means, he becomes distressed that only Banquo's children will be the future kings of Scotland, "for Banquo's issue have I filed my mind." This clearly reveals his selfishness and his greed for power. His deeply entrenched fear and insecurity regarding Banquo are emphasised when he compares Banquo to Mark Antony, Caesar's bugbear.