Ambition is a particular form of the sin of pride. Pride, in Shakespeare's day, was considered the root of all other sins. All sins, in other words, are rooted in excessive self-regard and in a failure to love and honor God. Macbeth's ambition is an extreme version of the more general failing of pride.
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:
The ambition of Macbeth and hiis wife leads directly to their destruction. They were going against the natural order and were aiming for a position which at the time was not deemed within their right to achieve.
Because this question is posted here in History, perhaps it is meant as a discussion of theme as it applies universally.
"Vaulting ambition" has been the great driving force of many a conqueror throughout history, and it is Macbeth's motivator, admittedly by him. Moreover, ambition feeds upon itself and Macbeth cannot stop when he begins his frenetic and bloody path. As Robert Frost writes in his "The Road Not Taken," he knows that "road leads to roard," Macbeth has formed his path by murdering Duncan. and extended it with the murder of Banquo. There is no turning back or stopping. Therefore, his unquenched thirst for power propels Macbeth to his tragic end.