Act I sc 3 has the following quote of Macbeth's aside:
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.
Those were his FIRST thoughts of taking over as king. Then he writes those thoughts out in a letter to his wife in the beginning of scene 5.
Next, Macbeth wavers back and forth while Lady Macbeth pushes him over the edge. His major change is when kills the servants himself (not part of the plan). Then he plans the death of Banquo and Fleance without telling her a thing. He truly has turned to "the dark side" at this point (Act III sc 1)
Our fears in Banquo
Stick deep; and in his royalty of nature
Reigns that which would be fear'd: 'tis much he dares;
And, to that dauntless temper of his mind,
He hath a wisdom that doth guide his valour
To act in safety. There is none but he
Whose being I do fear...