Macbeth has a moral compass when the play opens (this will disappear later as murder after murder hardens him). Therefore, he is torn by internal conflict in the first two acts. He is ambitious, so he listens when the witches tell him he will become both Thane of Cawdor and King of Scotland. When he becomes Thane of Cawdor shortly after the witches predict this, this whets his appetite for more power. He becomes convinced that he can become king and wants it now.
Yet, when push comes to shove, Macbeth's conscience and his wisdom come into conflict with his ambition. He feels deeply uncomfortable about murdering a good and "meek" king. Meek in Shakespeare's day didn't mean timid. It meant not violent or bloodthirsty. To murder any king, anointed by God to rule, is a terrible act, but murdering a good king is even worse.
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...
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