In the beginning of William Shakespeare's tragic play Macbeth, Macbeth is satisfied with the hope of becoming the king. In Act I, scene iii, Macbeth states that he will do nothing to force the throne.
If chance will have me king, why, chance
may crown me
Without my stir.
It is later, in Act I (scene vii) where Macbeth's ambition first becomes apparent.
With his surcease success; that but this blow
Might be the be-all and the end-all here,(5)
But here, upon this bank and shoal of time,
We'd jump the life to come.
Here, he admits to his understanding that with the king's death comes his own chance to claim the crown. Unfortunately, this initial ambition falters when pressured by his wife. When pressured, Macbeth refuses to consider his future as the king. Instead, he wishes to hold onto his title of Thane of Cawdor for a while longer.
Which would be worn now in their newest gloss,
Not cast aside so soon.
Here, Macbeth is questioning if he should really go though with murdering King Duncan. His desire to hold onto the title of Thane far exceeds his desire to become king.
After seeing the dagger (Act II, scene i), Macbeth realizes that he must go through with the murder of Duncan, given it is the only way he will be able to gain the throne.
As this which now I draw.
Thou marshal'st me the way that I was going,
And such an instrument I was to use.
Here, Macbeth the "way" he needs to go in order to gain the throne.
While Macbeth has flirted with his ambition, it is not until Act III, scene i where Macbeth beings to understand what he must do to keep his throne.
Rather than see that happen, come, Fate, into the arena,
And fight for me to the death!
It seems at this point, Macbeth understands that murdering Duncan was not enough. He must continue his murderous path. Fate, for Macbeth is paralleled with his ambition. If he had left all to fate, Macbeth would not have murdered Duncan in the first place. Now, worried about his crown, Macbeth's ambition has superseded the positive characteristics noted by Duncan in the beginning (when the title of Thane was given to him).
Therefore, Macbeth's ambitious nature becomes more powerful than he when he determines it necessary to murder Banquo and Fleance in order to keep his crown.