I wonder if the betrayal of Banquo by Macbeth is foreshadowed by their exceptional bravery on the battlefield. For if we consider their capacity for bloody valour as remarked upon by the Sergeant in his testimony in Act One Scene Two, then it is very likely that there exists between both warriors some form of competition? This fearless behaviour has served King Duncan well, as it has preserved his kingship as well as his kingdom. Yet the witches expose the competition that exists between the friends (and fellow soldiers) by addressing only Macbeth at first and ignoring his friend, which precipitates Banquo’s demand for a prediction too. Such a demand is met by a forecast that undermines the seductive triumphs of their predictions for Macbeth. It is as if they recognise the fragile balance of loyalty between the two soldiers and cunningly offer a vision of Banquo’s legacy that will render Macbeth uneasy and then impossibly jealous.
After all Macbeth’s lack of heirs is a source of much manipulation by Lady Macbeth. Indeed her malign references to her husband’s failures as a ‘man’( or potency) no doubt including his problems with producing a healthy heir, demolish any residue of noble reticence in her husband. So the existence of Fleance serves to remind Macbeth of the aridity of his masculinity and anticipates his sense of the worthlessness of his ‘ fruitless’ crown.