In Shakespeare's Macbeth, Lady Macbeth is certainly more at ease with killing Duncan than Macbeth. Lady Macbeth, upon hearing of the prophecies, prays to dark powers to make her less feminine, and fill her with evil intent:
Come, you spirits
That tend on mortal thoughts! unsex me here,
And fill me from the crown to the toe, top-full
Of direst cruelty... (I.v.41-44)
Later, when Macbeth begins to give reasons why they should not kill Duncan, we see into the darkest part of Lady Macbeth's soul, and it is chilling:
I have given suck, and know
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. (I.vii.60-65)
Lady Macbeth's part in the murder plans is to get the King's guards drunk. She expresses no fear at all: in fact, she is invigorated by what she has done:
That which hath made them drunk hath
made me bold;
What hath quench'd them hath given me fire. (II.ii.1-3)
After Duncan is dead, it is Lady Macbeth who, realizing Macbeth has brought the bloody daggers back to their room, returns them and spreads the King's blood all over the guards to make them look guilty of Duncan's murder.
Macbeth, however, hesitates. He offers reasons why he should not kill the King: Duncan is Macbeth's king; they are related; Duncan is enjoying Macbeth's hospitality (and Macbeth is honor-bound to protect any guest staying in his home); and, Macbeth really loves Duncan, who has also been good to him. Macbeth says that there is only one thing that drives him to do this horrific thing—ambition.
Macbeth tries to change his wife's mind about the murder, and she calls him a coward, saying he is less than a man. She harasses him until he agrees to kill Duncan. Macbeth's fear is palpable when he makes his way to the King's rooms and imagines that he sees daggers hovering before him, leading the way for him:
Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch thee. (II.i.41-42)
This shows how Macbeth's fear to do something that he knows is a sin against God has affected his mind: he is hallucinating.
We again see Macbeth mental state when he returns and the murder is done. Macbeth is obsessed that he could not say "Amen," while he listened to the guards pray. He hears a voice crying the he will "sleep no more." This also preys on his mind. He has, as well, mistakenly brought the murder weapons back to his rooms; when Lady Macbeth orders him to return the daggers, Macbeth is so distraught that he refuses to go back to the scene of the murder. His fear affects his behavior immediately afterward the act. Lady Macbeth questions her husband about the daggers:
Why did you bring these daggers from the place?
They must lie there. Go carry them, and smear
The sleepy grooms with blood. (II.ii.61-63)
But Macbeth is so overcome with fear that he cannot:
I'll go no more:
I am afraid to think what I have done;
Look on't again I dare not. (64-66)
We begin to understand also that Macbeth has deep guilt over what he has done. Hearing a knock, he says:
Wake Duncan with thy knocking! I would thou couldst! (93)