In Shakespeare's Macbeth, why does Macbeth want Banquo and Fleance murdered? If Macbeth believes in the witches' prophecies concerning himself, why does he think he can interfere with their prophecies concerning Banquo and his descendants?
In the modern-day, Macbeth, as portrayed in Macbeth, would certainly be a case study for the psychiatrist. Initially , he is a confident warrior, rewarded for his military exploits and clearly proud of his achievements as he is "a prosperous gentleman."(I.iii.73) It is not even unusual for him to feel "earnest of success" (132) at the prospect of being king or the realization that "nothing is but what is not."(140) He has recognized the unsettling coincidence that the witches first prophesy "commencing in a truth" (134) comes just before the confirmation of his elevated position as Thane of Cawdor and, whilst it does confuse him, he confirms that "The service and the loyalty I owe, In doing it, pays itself." (I.iv.22) He is, however, almost immediately conflicted as, on listening to Duncan's talk of Malcolm as The Prince of Cumberland, he begins to realize that he could "fall down"(49) unless he is able to "o'er-leap " this obstacle. After this, he constantly questions himself, and being insecure away from the battlefield, he does not command the same status from Lady Macbeth. She is able to undermine him and belittle him.
Macbeth places his trust in himself, Lady Macbeth and the witches so there can only be trouble when self-loving personalities such as this collide. Macbeth and Lady Macbeth bring out the worst in each other and superstition confounds the problem. Superstition has been known to encourage even rational people to perform unpredictably so the chances for Macbeth to think clearly are marginal. Hence, as soon as honor has deserted Macbeth; it is his loyalty to his king and "kinsman," not his morality that almost stops him; he can only resort to dishonorable means to secure his position and he schemes to murder Banquo and Fleance, having already eliminated Duncan, as they represent a hindrance in terms of his "vaulting ambition."(I.vii.27) He becomes angry at the thought that he has murdered "the gracious Duncan...only for them"(III.i.65- 67) so is almost duty-bound to remove them, revealing his conflict between duty, honor and his purported loyalty to the late Duncan.
The "great bond which keeps me pale" (III.ii.49) is the last of Macbeth's compassion. He effectively wants to steel himself against any weakness - human weakness - and does not even involve Lady Macbeth; somehow believing that more scheming and murder will make him more resolute in ensuring his kingdom. Macbeth is far from satisfied after Banquo's murder but is so far removed from reality that he is driving himself. He no longer worries about the witches claim to make "the seeds of Banquo kings"(III.i.69) as he will remove the problem.
Macbeth firmly believes in the witches prophesies regarding himself because he is typical of someone who believes he has moved away from the controls of mankind into another realm. Human control no longer governs Macbeth so he can overlook other prophesies as they become insignificant to the all-consumed Macbeth. The self-awareness that he shows when he intends to murder Duncan, and when he has nightmares and visions afterwards revealing his guilt, has forsaken him and been replaced by invincibility, thus securing his ultimate defeat, as there is no other way out for him.