There are a couple of times, before Act 3, that Macbeth thinks better of the murder of King Duncan which he will carry out in Act 2, scene 2. First, after seeing the witches, hearing their prophecies, and being swayed by them, he pulls back for a moment and says to himself (Act 1, scene 4):
If chance will have me king, why, chance
may crown me
Without my stir.
By this he means that he might become king without doing anything. Just relax.
Then in Act 1, scene 7, in his "If it were done when ’tis done, then ’twere well It were done quickly" soliloquy, Macbeth enumerates for all the reasons that he should not proceed in the killing. Then his wife interrupts him and he tells her:
We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon.
But she rejects what he is saying, chastises, demeans and excoriates him, and eventually Macbeth is left with this sheepish question: "If we should fail?"
She tells him to leave the rest to her, and they will not fail. He is then completely won over, and the rest is murder and more murder:
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.
To give you a sense of how far into killing he has gotten, there is this from Act 3, scene 4:
For mine own good
All causes shall give way. I am in blood
Stepp'd in so far that, should I wade no more,
Returning were as tedious as go o'er.
Strange things I have in head that will to hand,
Which must be acted ere they may be scann'd.
Any hesitation is long past.