Act III of "Macbeth" opens with Banquo in soliloquy: He reflects that he has acquired what the weird sisters predicted. He wishes that the prophecy of the witches about his sons also becoming kings will be fulfilled. But, upon the approach of Macbeth, he quiets himself, for he does not trust Macbeth.
Thous hast it now: King, Cawdor, glamis, all/As the weird women promised, and I fear/Thou play'ds most foully for 't. Yet it was said/It should not stand in thy posterity,/But that myself should be the root and father/Of many kings. If there come truth from them--/As upon thee, Macbeth, their speeches shine-/Why, by the verities on thee made good,/May they not be my oracles as well/And set me up in hope? But hush, no more! (III,i,1-10)
After Banquo departs, Macbeth then speaks a soliloquy:
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 feared. 'Tis much he dares;/And, to that dauntless temper of his mind,/He hath a wisdom that doth guide his valor/To act in safety. There is none but he Whose being I do fear: and under him/My genius is rebuked, as it is said/Mark Antony's was by Caesar.(III,i,48-57)
Now Macbeth is worried about Banquo, fearing both the wisdom and progeny of Banquo. He ponders that his murder of Duncan may have been "For Banquo's issue" and not for himself and his sons. So in order to elimate any obstacles, Macbeth hires two murderers to kill Banquo and his son. He tells these murderers that he cannot be connected to this murder as he must remain friends with some who also like Banquo. They agree to do the deed. Macbeth says,
It is concluded: Banquo, thy soul's flight,/If it find heaven, must find it out tonight. (III,ii,141-142)
These words of Banquo and, especially, Macbeth indicate that the friendship between the two men has been dissolved by the influence of the spiritual world and by the ambition in both men's hearts.