When Macbeth first considers whether or not to kill the king, his conscience appears to win him over and convince him that he should not do it. He reasons to himself that Duncan does not deserve to be murdered, because he has been "so meek," meaning gentle, and because he has been "So clear in his great office," meaning that he has been such a good king. Macbeth, thus convinced, tells his wife, "We will proceed no further in this business," and says that King Duncan has "honour'd" Macbeth. The implication here is that it is unconscionable to consider the "business" of murdering the king when that king has been so good as to heap honors upon Macbeth.
However, Macbeth's conscience is soon—and rather easily—overturned by his wife. Lady Macbeth accuses her husband of being cowardly and womanly. She asks him if he will be happy to "live a coward," and she tells him that only when he commits to the murder will he be able to call himself a man.
At first, Macbeth's conscience tries to hold firm. Macbeth insists, "I dare do all that may become a man; / Who dares do more is none." Lady Macbeth then tells him that he is breaking a promise that he made to her and that she would sooner have "dash'd the brains out" of their child rather than break a promise she had made to him. This seems to be enough to break the hold of Macbeth's conscience. He asks, "If we should fail?" The implication here is that he is after all prepared to murder the king, as long as he can be confident of getting away with it.