In Shakespeare's "Macbeth," the news that Fleance, Banquo's son, has escaped is particularly distressing for Macbeth because it jeopardizes his own power and security. Macbeth has become king through his murderous actions, having killed King Duncan and orchestrated the murder of Banquo, who posed a threat due to the witches' prophecy that Banquo’s descendants would inherit the throne.
Macbeth is troubled by the fact that Fleance is still alive because the prophecy suggests that Banquo's lineage will eventually claim the throne. This means that Macbeth's rule is unstable, and his crown may be at risk if Fleance grows up to challenge him. Additionally, Macbeth is increasingly paranoid and realizes that he must eliminate any potential threats to his power, which includes Fleance, the one remaining heir to Banquo. The escape of Fleance indicates that Macbeth's plans are not as foolproof as he believed, and it deepens his sense of insecurity and fear about the consequences of his violent actions.