"I have no spur," says Macbeth in Shakespeare's Scottish play, "but only vaulting ambition, which o'erleaps itself and falls on th' other" (2.1. 25-28). Ambition as both theme and motive is the play's spur also, as Shakespeare illustrates both the positive and negative effects of ambition in the different ways his characters handle it.
Negative to Murderous
Macbeth's ambition to be king poisons his outlook before the play even begins. Harold Bloom speaks of the thane's "proleptic imagination": when the witches' prophesy of his promotion comes true, he immediately sees King Duncan dead (1.3.51-52). The murder that follows is almost superfluous; in Macbeth's imagination, he has already killed Duncan. Similarly, when Lady Macbeth hears of his promotion in a letter (1.5.40-54), she unhesitatingly calls on the powers of darkness to increase her feminine cruelty; she knows she must persuade her husband to murder, not exactly a wifely duty.
Negative to Nihilistic
The scenes that follow reinforce the effects of the Macbeths' ambitious negativity: we never see them swear a pact -- it may have been pledged years before -- but Lady Macbeth declares she would "dash the brains out" of an infant had she sworn as Macbeth apparently has (2.1.54-59). Their deep-seated ambition infects their entire marriage; "negative" hardly begins to describe their nihilistic loss of normal intimacy, parenthood and satisfaction as they reject "golden opinions" from others (2.1.32). They proceed to madness, death and the epitaphs of "dead butcher and his fiendlike queen."
Cowardly to Courageous
The hungrily negative effects of the Macbeths' ambition are fairly obvious; less straightforward is the positively charged ambition of young Malcolm. He does not begin promisingly, running like a rabbit when murder takes his father's crown (3.3.135-139); however, by Act IV he is raising an army to take back his country and throne, saying "Macbeth is ripe for shaking" (4.3.237-238). The effects of his positive ambition are noteworthy: he allies himself with King Edward of England, whose royal touch is holy and healing (4.3.149-152).
A Country's Love or Hate
The most telling point that compares Macbeth's negativity with Malcolm's positive effect is the result of their interviews with underlings. Macbeth, to inspire two murderers, lies about the innocent Banquo (3.1.75-82); Malcolm, to test Macduff's loyalty, lies about himself before revealing the truth (4.3.57-135). Macbeth dies surrounded by enemies; all England rises to assist Malcolm. The love, or hate, of an entire country is the most pronounced example of the effects of positive and negative ambition.