In Macbeth's opening soliloquy, he lists the great many reasons he has not to go through with Duncan's murder: he is Duncan's host, his subject, his friend.  Further, killing Duncan doesn't necessarily mean he gets the throne; more action will be required to achieve this.  He also knows that Duncan is a good king and that Macbeth will compromise his soul by killing him.  However, he finishes with his one reason to go forward: his "Vaulting ambition" (1.7.27). 

Even his ambition, though, is not enough.  When his wife enters, he tells her, "We will proceed no further in this business" (1.7.31).  He has resolved not to kill the king as a result of the multitudinous points against it.  Then again, Lady Macbeth begins to work on him: cajoling, persuading, and insulting him.  She says that he won't be a man if he doesn't go through with the murder and how she'll count him as disloyal to her if he goes back on his word.  At last, she convinces him.  However, Macbeth's ambivalence is evident by the internal conflict he voiced during the soliloquy and by his resulting resolve to cancel their plans before flip-flopping and acquiescing to his wife's demands for loyalty.