What bad thought enters Macbeth's mind?
After the captain reports to King Duncan of Macbeth's fearlessness in battle in the opening scene, Duncan exclaims "O valiant cousin!" and then orders the death of the traitorous Macdonwald, announcing that he will bestow the title of Thane of Cawdor upon Macbeth. (1.2)
In Scene 3, Macbeth and Banquo, having returned from the battlefield, happen upon three witches who offer their predictions to them, by greeting Macbeth by saying "Hail, Thane of Cawdor!" and by telling Banquo that his sons will be kings. Macbeth is intrigued by these predictions, but Banquo advises him not give credence to the preternatural creatures, telling him,
The instruments of darkness tell us truths,
Wins us with honest trifles, to betray's (1.3.134-135)
Still, Macbeth "yields to that suggestion/Whose horrid image doth unfix [his]hair," especially when he is greeted by Ross and Angus as Thane of Cawdor, who make prediction of the three witches come true. Then the idea of murdering Duncan enters the mind of Macbeth and he contemplates that
Are less than horrible imaginings:
My thought, whose murder yet is but fantastical (1.3.148-150)
Thus, it is Macbeth's "vaulting ambition" that makes him desirous of power to the point that he contemplates murdering his cousin King Duncan so that he, then, can become king.
My current fears
Are less than horrible imaginings.
My thought, whose murder is still only a fantastic idea,"