Macbeth provides the reasons why he fears Banquo in Act lll, scene l, during his monologue. He declares that to be king means nothing unless he is safe and secure in his new role. He acknowledges that he harbors a deep fear for Banquo for the following reasons:
- Banquo is noble and good and it is this quality that makes him a most powerful adversary.
- He is courageous and strong-willed and would take risks for the sake of good.
- He is wise enough to know that he should not be impetuous and put himself in danger. He should ensure his own safety.
- With Banquo around, Macbeth feels that his marvellous plot in killing Duncan could be exposed.
- Macbeth will always be deemed inferior to Banquo, for as long as Banquo lives, just as much as Antony was said to be when compared to Caesar.
- Macbeth noted Banquo's reaction to the weird sisters' predictions and was aware that he was very critical and skeptical of what they had said. It seemed to him as if Banquo resented the witches' good tidings to him and even scolded them about it.
- Banquo insisted that the witches provide him with a prediction also, probably to test the veracity of their prophecy.
- Macbeth was disgusted by the fact that the witches predicted that Banquo's heirs would be kings, ignoring the fact that if he were king, his offspring were supposed to inherit the throne. The implication is obvious - Macbeth's rule will end with him for he would leave no heirs.
- Macbeth, as such, resents the fact that he had committed such a foul deed as to murder Duncan for the benefit of Banquo's heirs. He hates the fact that he had to suffer so much turmoil and unsettle his peace for them whilst his legacy would die with him. He is so overwhelmed by this thought that he utters a furious outburst in which he expresses his disgust.
In the end, Macbeth decides that to prevent what the witches' had predicted, Banquo has to be killed. Banquo is too great a threat:
Rather than so, come fate into the list.
And champion me to the utterance!
He alone should benefit from the witches' predictions. He later affirms the fact that Banquo must die after he had spoken to the assassins whom he had hired to kill Banquo and his son, Fleance. Fleance has to be killed to negate the witches' prediction regarding Banquo's heirs.
It is concluded. Banquo, thy soul's flight,
If it find heaven, must find it out to-night.
Macbeth is so determined to have Banquo killed that he has plotted to have the pernicious deed performed that same night.
Macbeth's suspicions arise not only from what he has observed and what he knows about Banquo, but also from how Banquo had responded to his suggestion, when they speak in Act ll, scene l:
If you shall cleave to my consent, when 'tis,
It shall make honour for you.
So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counsell'd.
Macbeth is clearly suggesting that Banquo should support him and that he would reward him when the time comes. Banquo's cryptic reply suggests that he would lose none of the honor he already has if he chooses to enhance it as long as he retains his loyalty to his king (Duncan). It is obvious that Banquo would never betray his liege.