While there is also a strong theme of fate present in Macbeth, it is without a doubt his ambition (and his wife's as well) that drives them to usurp the throne. It drives him to commit horrible acts, plunges the kingdom into conflict, and ultimately brings about his demise. In Act 1, scene 7, Macbeth himself admits as much as he is about to murder Duncan, a man with whom he has absolutely no quarrel:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself,
And falls on th'other. . . .
Only ambition drives him to do what he is about to do, an act he realizes is wrong. By evoking the imagery of a horse, Shakespeare seems to be suggesting that Macbeth is not in control of his own ambition.
There is much on enotes concerning the topic of Macbeth in particular, and ambition in general: