the play.

Initially, it is Lady Macbeth who decides Duncan must be killed so her husband can become king. In one of her soliloquies, Lady Macbeth states her fear that Macbeth does not possess the malevolence to do what is necessary to take the throne—in other words, to kill Duncan:

Yet I do fear thy nature;

It is too full o' the milk of human kindness

To catch the nearest way: thou wouldst be great'

Art not without ambition, but without

The illness should attend it: what thou wouldst highly,

That wouldst thou holily; wouldst not play false,

And yet wouldst wrongly win: thou'ldst have, great

Glamis,

That which cries 'Thus thou must do, if thou have it;

And that which rather thou dost fear to do

Than wishest should be undone.

She knows that her husband is kind-hearted and will have to be encouraged and persuaded to kill Duncan. She waits for him to return home so she can talk him into murdering the king:

Hie thee hither,

That I may pour my spirits in thine ear;

And chastise with the valour of my tongue

All that impedes thee from the golden round,

Which fate and metaphysical aid doth seem

To have thee crown'd withal.

She asks the spirits to give her the strength and resolve to move forward with her plan to help her husband take the throne. She does not want guilt to prevent her from realizing her goals. Macbeth is hesitant at first and contemplates whether or not he should kill Duncan. Soon after killing the king, Macbeth is overcome by guilt over what he has done. He hears a voice telling him that he will "sleep no more." When Lady Macbeth tells her husband to return the daggers to Duncan's room, Macbeth replies, "I am afraid to think what I have done; / Look on't again I dare not." Macbeth is too upset to look upon Duncan again and his wife tells him he is a coward.

While Lady Macbeth is initially the instigator and Macbeth the follower, this changes throughout the course of the play. Although it is her idea to have Duncan killed, Lady Macbeth is later haunted by guilt. She sleepwalks, hallucinates that there is blood on her hands, and compulsively washes her hands to rid herself of the blood, which symbolizes her guilt:

Out, damned spot! out, I say!—One: two: why,

then, 'tis time to do't.—Hell is murky!—Fie, my

lord, fie! a soldier, and afeard? What need we

fear who knows it, when none can call our power to

account?—Yet who would have thought the old man

to have had so much blood in him.

As Lady Macbeth's is slowly driven insane with guilt, her husband becomes increasingly comfortable with killing to secure and maintain his title. The witches prophesize that Banquo's descendants will become king. To prevent this prediction from coming to pass, Macbeth decides to have his friend and fellow soldier killed, along with his family, including his son, Fleance. Macbeth makes this decision on his own and does not even inform his wife of his plans. When she asks him what he is planning to do, he tells her that he cannot share that information with her until the deed is done:

Be innocent of the knowledge, dearest chuck,

Till thou applaud the deed.

Whether he excludes her from his decision to protect Lady Macbeth or because he fears she will not approve, Macbeth no longer needs encouragement or suggestions from his wife to commit evil, violent acts. He is now capable of devising and carrying out such acts himself.

Early in the play, Lady Macbeth instigates violent acts and is so consumed by ambition, she talks her husband into killing the king and taking his throne. During this time, Macbeth is hesitant and later overcome by guilt over what he has done. These roles reverse as the play progresses. Lady Macbeth, who previously scolded her husband for feeling guilty, is haunted by guilt over her role in Duncan's murder. She becomes insane and ultimately ends her life because of her guilt.

Early in the play, Macbeth is reluctant to kill Duncan and is immediately filled with guilt after killing him. He is kind-hearted initially and needs to be goaded into killing Duncan. As the play progresses, Macbeth's guilt and kindness are replaced by a ferocious desire to protect his position as king. He no longer needs his wife to talk him into committing violent acts; he does this himself. His wife has no part in his decision to kill Banquo and Fleance; in fact, she is not even privy to her husband's plans.

Ambition is one thing, murdering out of fear is quite another.

It is quite correct to say that, in the beginning, after he is tempted by the witches, Macbeth thinks about killing Duncan in order to become king himself (Act 1, scene 3):

My thought, whose murder yet is but fantastical,

Shakes so my single state of man that function

Is smother'd in surmise, and nothing is

But what is not.

But Lady Macbeth is right: her husband is not cruel enough to kill for what he sort of wants. No, it is Lady Macbeth's "vaulting ambition" that ultimately propells Macbeth to do the deed. In fact, he knows of her desires and tells her pointedly, in Act 2, "We will proceed no further in this business." Then he gives her all kinds of very good reasons why they should not kill the king, but she prevails and manupulates him into doing the murder.

Macbeth is not ambitions, he is weak. Yes, the great and brave warrior is weak compared  to his overbearingly ambitious wife. After the murder, fear overtakes Macbeth, and what looks like a wll to power is not ambition but a ferocious attemp to be safe at all costs: "For mine own good/All causes shall give way."

Macbeth's ambition is the driving force in the play.  Certainly Lady Macbeth is also ambitious and thus persuades her husband to do whatever it takes to secure his position as King; however, it is Macbeth's ambition that drives the later events in the play.  In the first act, Lady Macbeth says that Macbeth is not without ambition but that he lacks the drive to put his ambition into reality.  So, she helps him carry out Duncan's murder.  But after this is done, Macbeth is so driven to cover up his crime and continue his reign as King that he has many other people murdered including Banquo and Macduff's family.  Lady Macbeth knows nothing of these murders while Macbeth is planning them, so her ambition is not a driving force for the later murders.  Further, Lady Macbeth eventually repents her role in these crimes while Macbeth vows to fight to the bitter end.  Thus, Macbeth's ambition is a greater driving force in the play.

With the story of the Garden of Eden and with Macbeth, I think too much blame is placed on the woman. Lady Macbeth might instigate the murder of Duncan, but they are both essentially at fault. In Act 1, Scene V, Lady Macbeth begs to "unsex" herself implying that she must become unnatural or more masculine in order to go through with the murder plot. This scene makes her seem more evil (especially juxtaposed to the "unnatural" witches) but she feels she must become less stereotypically feminine (the stereotype being the weaker sex) in order to be willfully strong enough to go through with the murders. This could be interpreted to be more about the roles of women and men than it is about Lady Macbeth asking to become unnatural or evil. 

Even if the majority of blame is placed on Lady Macbeth for Duncan's death, it is Macbeth who orders the killing of Macduff's family as well as hiring murderers to kill Banquo and Fleance. 

Both Macbeth and Lady Macbeth share the responsibility. In their distorted bond of marriage they feed off of each other. When one is hesitant, the other encourages him/her. When Macbeth hesitates about killing Duncan, Lady Macbeth chastises him. When Macbeth hallucinates, Lady Macbeth sends him away to avoid any suspicion from others. 

They are also both guilt-ridden, proof that they both feel responsible for the murders. Macbeth hallucinates, seeing Banquo's ghost, because of his guilt and fear. Lady Macbeth sleepwalks and eventually commits suicide, also out of guilt and fear of being discovered as being partly responsible for the sequence of murders. 

Macbeth never talks about his wife's ambitions, but she talks about his. Her soliloquy in Act 1, Scene 5 provides a good means to evaluate the relative strengths of their ambitions.

Glamis thou art, and Cawdor, and shalt be
What thou art promised. Yet do I fear thy nature;
It is too full o’ the milk of human kindness
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it.
.......................................................................
Hie thee hither,
That I may pour my spirits in thine ear,
And chastise with the valor of my tongue
All that impedes thee from the golden round,
Which fate and metaphysical aid doth seem
To have thee crown'd withal.

Lady Macbeth obviously believes that her own ambition is stronger than that of her husband and that he will not act to become king unless she uses her persuasive powers. She is probably correct. We see throughout the beginning scenes that Macbeth has all sorts of misgivings about murdering Duncan. This should indicate that he is, as his wife says, not sufficiently ambitious to overcome his doubts and scruples. She is obsessed with the vision of becoming queen, but she cannot become queen until her husband becomes king. She overrides all his objections to going through with the plot to kill Duncan. She is so ambitious that she is blinded by her ambition. Macbeth is not blinded because he is not as strongly motivated. He sees all the possibilities of something going wrong and tries to explain them to her--but she won't listen. She believes or pretends to believe that all her husband's arguments against committing the murder are prompted by fear, which she stingingly calls cowardice.  It seems likely that Macbeth would not have killed King Duncan if his wife had not taken the lead, and this in itself seems to prove that she is more ambitious than her husband.

Lady Macbeth is more ambitious, though her ambition has to be largely vicarious. These are her first words in the play, after reading her husband's letter:

Glamis thou art, and Cawdor; and shalt be
What thou art promised: yet do I fear thy nature;
It is too full o' the milk of human kindness
To catch the nearest way: thou wouldst be great;
Art not without ambition, but without
The illness should attend it: what thou wouldst highly,
That wouldst thou holily...

First, there is the exclamation of triumph, not for herself but for him. Despite her great strength of character, Lady Macbeth knows that she is only the wife of Glamis, now the wife of Cawdor, and that the highest hope she can have is to be the wife of and influencer of the King. To bring herself high, she must bring him higher. Her next concern is that Macbeth is too good a man to achieve what he must. She must use him, almost as a pawn, while propelling him to the throne.

Before Macbeth enters in act 1, scene 5, Lady Macbeth has made her famous appeal to the spirits "That tend on mortal thoughts" to unsex her and make her more cruel even than she is already. Macbeth is later to grumble about having damned himself by trafficking with the powers of darkness, but Lady Macbeth wants to summon them to her, whatever the consequences, if it will help her to achieve her ambition.

Throughout the play, particularly in II.ii, when Macbeth has just killed Duncan, and in III.iv, when Banquo's ghost appears, Lady Macbeth has to cover for her husband's inadequacies and prop him up in his position. To Macduff, Malcolm, Banquo, eventually to almost everyone, Macbeth is a monstrous figure, but to his wife he remains the man who is continually failing at a task she would perform so much better. The rewards of her ambition are not so great. To be queen is less than to be king, but Lady Macbeth's ambition and courage are much greater than Macbeth's until her final breakdown while sleepwalking in act 5, scene 1, after which we never see her again. The reason for this absence is clear: in defiance or in repentance, she would outshine Macbeth and take the focus away from his final desperate conflict with Macduff.

Your question has been argued for centuries, so my answer is certainly not absolute. What I will do is offer some evidence on both sides of the argument.

In favor of Macbeth’s being the more ambitious of the two, some critics point out that Macbeth’s “murderous resolve” increases as the play progresses, while Lady Macbeth weakens to the point of insanity and suicide. On the other hand, it is not until Lady Macbeth taunts her husband into going through with the murder of Duncan that Macbeth overcomes his reluctance. It is Lady Macbeth who decides on the time, place, and method of killing Duncan.

My opinion is that the overweening ambition that originates with Lady Macbeth infects her husband. In the end, it is Macbeth who commits the murders and faces his own end with more courage than his wife faced hers.