Final answer:
Macbeth is upset about Fleance's escape as it threatens the prophecy of Banquo's heirs becoming kings, which would undermine his own kingship.
Step-by-step explanation:
Macbeth is upset when the murderers reveal that Fleance had not been killed because he worries that the prophecy of Banquo’s descendants becoming kings is coming true. This is illustrated in the play as Macbeth's ambition and desire for power lead him to fear any threat to his kingship. The importance of prophecies and Macbeth’s reaction to them is a central theme throughout the play.
The prophecy made by the witches suggests that Banquo’s lineage will inherit the throne, which undermines Macbeth’s power and creates a sense of paranoia within him. Although Fleance is not presented as a direct threat in terms of power at that moment, the potential of the prophecy being fulfilled through him makes Macbeth anxious and distressed.
Fleance's escape signifies the possibility of Banquo's heirs continuing their bloodline and eventually challenging Macbeth's position as king. It is this fear, rather than personal feelings toward Fleance, jealousy, or concerns about military threats, that upsets Macbeth with the news of Fleance's survival.