It is not only the threat that Fleance's escape from death poses for Macbeth, but it is also the fact that the witches' predictions have verity. Now, for Macbeth "Nothing is what is not," and Macbeth's anxiety and paranoia increase, for if Fleance lives, so may Birnam Wood move.
Macbeth's perception of reality and fantasy seem to merge now and as "blood will have blood," the more that Macbeth becomes mired in his bloody deeds, the more horrified and violent he becomes as the pressure to dissemble and to eliminate his enemies presses upon him. For, after he learns that Fleance has escaped death, Macbeth, in his horror, begins to see the ghost of Banquo and imagines him seated at the table with the guests. To the dismayed guests, Lady Macbeth excuses her husband's behavior as an old childhood delusion that recurs on occasion so they depart. After they leave, Macbeth tells his wife that his fears come from lack of experience and will harden with further malevolent deeds
...My strange and self-abuse
Is the initiate fear that wants hard use.
We are yet but young in deed.
Furthermore, he decides to visit the witches again.
Fleance's escape is necessary for the fullfilment of the witches prophecy. Banquo gets murdered, Macbeth must face the inevitable, that Banquo's heirs will command the throne one day. Fleance, being Banquo's son will set this process in motion with his actions in the future.
Fleance's escape insures that Macbeth will never be satisfied with his crown, he will never feel that he has achieved true success. He already feels that he has murdered for Banquo's heirs. He holds an empty septer and a fruitless crown. Macbeth has no children, so therefore, he feels that his reign will be too short and constantly threatened because of Fleance.
Initially when Macbeth and Banquo met the witches, the witches told the men their future. To Macbeth, they promised the crown; to Banquo, they promised that his son would inherit the crown. Therefore, Macbeth wants Fleance killed, so that he would not feel threatened by the fact that Fleance is destined to be king. Because Fleance escapes, Macbeth cannot be content with his power. Also, his escape reinforces the witches' prophecy. Macbeth's ambition has yet to overpower destiny.
The witches prophesied that Banquo “shalt get kings.” As a result, all descendents of Banquo pose a threat to Macbeth’s reign. Fleance’s escape terrifies Macbeth. At this news, he states, “Then comes my fit again: I had else been perfect.” His survival is a symbol of good existing in the face of evil, including his father Banquo’s murder at his friend Macbeth’s order.
The witches’ prophecies seem to affect everyone who knows of them, including Banquo, who worried these “truths” were told simply “to win us to our harm.” He agrees to talk to Macbeth about the “weird sisters,” for they have been weighing on his mind. Banquo suspects Macbeth of killing Duncan and possibly even contemplates murder himself, though it is an urge Banquo manages to suppress.
Even though Fleance’s flight appears to signify hope, one question is how Fleance or his children become kings if Duncan’s sons rule after him. Perhaps Fleance does not play “most foully for't” as Macbeth does, but he may very well follow the cycle begun by the Thane of Cawdor before Macbeth. If both that thane and Macbeth betrayed the king, one wonders how Fleance’s survival will contribute to or break this cycle of treachery.