In Act III, Scene 1, Macbeth seems to have lost all conscience and succumbed to his lust for power.
Beforehand, Macbeth has faltered at the idea of killing King Duncan in order to become king more quickly and prevent Duncan's son Malcolm from having any claim to the throne. In Act I, Scene 7, in a soliloquy, Macbeth has had compunctions against committing the murder because Duncan is his kinsman and he is virtuous. Then, since he is going to be Duncan's host, Macbeth has felt he should be kind and not murder the king. Thirdly, if the king dies, the country will mourn and the news of the horrible deed will spread throughout. Nevertheless, Macbeth concludes that the only motivation he has is his "vaulting ambition which o'erleaps itself/And falls on th'other--"(1.7.27-28)
So Macbeth has killed Duncan because he has also been coaxed by Lady Macbeth to commit the murder after she questioned his manhood. After the murder, however, he is overcome with guilt as he has imagined a voice cry "Macbeth doth murder sleep" (2.2).
But, now, in Act III, Scene 1, Macbeth seems to have become more callous and ambitious. When Banquo tells him that he will be gone for two hours, but will arrive at Inverness for the banquet shortly thereafter, Macbeth hires two murderers to eliminate Banquo as well as his son Fleance. He is even so treacherous as to wish Banquo "While then, God be with you!"
Macbeth has hired these murderers because he feels that if he does not kill Banquo, he has murdered "the gracious Duncan" only so Banquo's sons could become king. Therefore, he challenges fate and vows that he will fight it to the death.