Quote 1
MACBETH [Aside]
If chance will have me king, why, chance may
crown me,
Without my stir. (1.3.157-159)
Here, Macbeth briefly decides to let "chance" take its course rather than fighting things, or, you know, murdering his noble king. Piece of advice, Macbeth: go with this line of thought. But if "chance" is the same as "fate," then it seems to amount to the same thing—and it's not good for Macbeth. Or Duncan. (Decent for Malcolm, however.)
Quote 2
MACBETH [Aside]
The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be,
Which the eye fears, when it is done, to see. (1.4.55-60)
Uh-oh. Once he learns that King Duncan has named Malcolm the Prince of Cumberland and heir to the crown of Scotland, Macbeth isn't content to wait around for "chance" to intervene. He decides that he must take action, or "o'erleap" the obstacles in his path to the throne. By murder. Well, this seems pretty willful to us.
Quote 3
MACBETH
Prithee, peace:
I dare do all that may become a man;
Who dares do more is none. (1.7.50-52)
When Macbeth tries to insist that the murder plot is off, Lady Macbeth needles him (and makes a few impotence jokes) until he finally gives in. That's right—gives in. Saying "I dare do all that may become a man" sounds a lot like he's made a decision.