Many scholars note that Macbeth and Lady Macbeth start out with differing attitudes, and then change places by the end of Shakespeare's play, Macbeth.
Macbeth has fought valiantly for king and country, and Duncan has rewarded him richly—with promises of more to come! His "vaulting ambition" motivates him not for the sake of money, but for power. He struggles even with the inkling of killing Duncan, as soon as the witches make their predictions—for Duncan is alive...how can Macbeth be king if Duncan lives?
In Act One, scene five, Lady Macbeth reads aloud Macbeth's letter that describes his meeting with the witches
'They met me in the day of success, and I have
learned by the perfectest report, they have more in them
than mortal knowledge. When I burned in desire to question
them further, they made themselves air, into which
they vanished...' (1-5)
Macbeth's letter indicates his awe over the witches' words. He is amazed that their prediction regarding his new title taken from Cawdor has come true. When this happens, he writes to his wife that he cannot help but consider becoming king someday. However, he does not imply a desire to take steps to make it come true. He simply shares, with the woman he loves, the news that one day she will be queen. That is all.
It is in this scene, however, that Lady Macbeth worries about Macbeth, believing he is not evil enough to do what is necessary to take the throne; her attitude is very different than her husband's:
Yet do I fear thy nature;
It is too full o’ the milk of human kindness
To catch the nearest way. (14-16)
She begins to make plans of her own. By the time the messenger arrives informing her that the King is staying at their castle that night, Lady Macbeth is intent upon killing Duncan. She refers to the raven, an omen of death, and speaks of Duncan's "fatal entrance"— his last entrance anywhere:
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. (39-41)
Note that Duncan enters under her battlements. Then she calls on dark spirits to remove womanly softness from her, and make her heart vicious so she can kill!
On the other hand, Macbeth balks at the idea of taking Duncan's life; in scene seven, he wishes that it were done and over with, for killing Duncan goes against his nature:
If it were done when ’tis done, then ’twere well
It were done quickly. (I.vii.1-2)
He goes on to note the reasons he should not kill the King: he is a guest; he is related; he is Macbeth's king; and, he is a good man—Macbeth is sure that even heaven will cry out in horror over the murder. He goes as far as to tell his wife they won't continue with the plan:
We will proceed no further in this business... (34)
He wants to enjoy Duncan's honors, and the praise from others. It is here we see how warped Lady Macbeth is. She insults Macbeth's manhood; accuses him of making empty promises; and, says she could and would murder an infant if she had promised to do so:
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this. (60-65)
Lady Macbeth is truly evil; she badgers him until he agrees.
But by the end, Macbeth murders with little or no provocation. His wife sleepwalks, reliving the murders; insane, she later takes her life. Macbeth's speech at the start infers doubt; his wife's, conviction!