What does the "crown" metaphor compare to in Macbeth?
There are many symbols in Macbeth that support the tragedy and Macbeth's fatal flaw and even Macbeth recognizes his own potentially damaging and "vaulting ambition"(I.vii.27) At first, Macbeth may be content to let "chance (may) crown me Without my stir" (I.iii.143) although "nothing is but what is not."(141) The "crown" represents the natural order and is usually associated with authority, kingdom and, certainly in Shakespeare's day, with God's power. It is significant that Macbeth's visions involve the witches - unnatural order - questioning the true power of "the crown."
The "fruitless crown" (III.i 60) alludes to Macbeth's state of mind. It seems that he is still not content even though he is "King, Cawdor, Glamis, all"(III.i.i) He is already feeling threatened by Banquo and his "line of kings" (59) and he feels his future is uncertain as he has a "barren sceptre,"(61) having no heirs to pass the line through and cannot bear to think that he may have killed "the gracious Duncan"(65) and secured Banquo's future. As the crown represents natural order and Macbeth knows his ascendancy is undeserving; hence he cannot relieve himself of his fears.
In Act IV, scene i, Macbeth is again haunted by visions of the "crown" such as it appears in the apparitions as it "doth sear mine eye-balls."(113) Macbeth recognizes the image of the crown as having the potential to destroy him but, even despite his confidence that he shall "never vanquish'd be"(93), he still feels threatened by Banquo's potential, even after his death. It seems that Banquo will get the better of Macbeth regardless as the "blood-bolter'd Banquo smiles upon me."(123) Again, it is Macbeth's seemingly unconscious knowledge that he has acquired the "crown" through evil and despicable means that ensures he is haunted by his fears and unable to rest in his kingdom.
The crown metaphor thus refers to natural ascendancy and the order of things, such as they are supposed to be.