By scene one of act three of Macbeth by William Shakespeare, Macbeth is firmly established as king of Scotland. Of course he usurped the throne by violence--while Duncan was a guest in his home, no less--and has since been suffering from a guilty conscience. Something else has begin to happen, as well; Macbeth has begun to worry about his friend Banquo.
Banquo is the only other person (other than Lady Macbeth) who knows about the witches' prediction regarding Macbeth as king. Macbeth has a reason to be worried because, though Banquo has not said anything yet, he could do so at any time, jeopardizing Macbeth's kingship.
The idea of the Divine Right of Kings is something an Elizabethan audience would have been well aware of, and killing a king was tantamount to (equal to) condemning oneself to eternal damnation. Macbeth knew the cost of his actions and was willing to pay it. What is becoming quite galling (irksome) to Macbeth, however, is the fact that he is the one who has sacrificed his soul and his conscience by killing the king but, according to the predictions, Banquo's son is going to inherit the throne.
There is none but heWhose being I do fear.... He chid the sistersWhen first they put the name of king upon meAnd bade them speak to him. Then, prophetlike,They hailed him father to a line of kings.Upon my head they placed a fruitless crownAnd put a barren scepter in my grip,Thence to be wrenched with an unlineal hand,No son of mine succeeding. If ’t be so,For Banquo’s issue have I filed my mind;For them the gracious Duncan have I murdered;Put rancors in the vessel of my peaceOnly for them; and mine eternal jewelGiven to the common enemy of man,To make them kings, the seed of Banquo kings!