Well, first he does, then he doesn't, then he does. Sort of like a fish nibbling at the bait...
When Macbeth first meets the witches, he is overcome by the possibilities presented to him (Act 1, scene 3):
[Aside.] Two truths are told,
As happy prologues to the swelling act
Of the imperial theme!—I thank you, gentlemen.
[Aside.] This supernatural soliciting
Cannot be ill, cannot be good. If ill,
Why hath it given me earnest of success,
Commencing in a truth? I am Thane of Cawdor.
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? Present fears
Are less than horrible imaginings:
My thought, whose murder yet is but fantastical,
Shakes so my single state of man that function
Is smother'd in surmise, and nothing is
But what is not.
Yes, he is moved to believe in them and the future that they offer him and is already thinking of murder. Then, quickly, he has second thoughts and decides maybe things will happen the way they happen; he'll just wait and see:
[Aside.] If chance will have me king, why, chance may crown me
Without my stir.
Along those same lines, later in scene 7, Macbeth figures he has it good enough, he's still the captain of his fate, and the heck with what the witches say; he'll just keep things as they are:
If it were done when ’tis done, then ’twere well
It were done quickly. If the assassination
Could trammel up the consequence, and catch,
With his surcease, success; that but this blow
Might be the be-all and the end-all here,
But here, upon this bank and shoal of time,
We'd jump the life to come. But in these cases
We still have judgement here, that we but teach
Bloody instructions, which being taught return
To plague the inventor. This even-handed justice
Commends the ingredients of our poison'd chalice
To our own lips. He's here in double trust:
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself. Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels trumpet-tongued against
The deep damnation of his taking-off,
And pity, like a naked new-born babe,
Striding the blast, or heaven's cherubin horsed
Upon the sightless couriers of the air,
Shall blow the horrid deed in every eye,
That tears shall drown the wind. I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps
And falls on the other—
Then in comes Lady Macbeth and she chastises him with her tongue and convinces him to believe again in the witches' prophecy and promise of greatness. Finally, he agrees:
Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males. Will it not be received,
When we have mark'd with blood those sleepy two
Of his own chamber, and used their very daggers,
That they have done't?
So, he's back again and says, at the end of the act:
I am settled, and bend up
Each corporal agent to this terrible feat.
Away, and mock the time with fairest show:
False face must hide what the false heart doth know.
Now he's caught.