Both Macbeth and his wife display their ambitious nature by plotting and participating in King Duncan's assassination. Despite Macbeth's initial reluctance to commit regicide, his comments regarding how he will attain the title of king reveal his ambition. Immediately after Macbeth learns that one of the prophecies has come true, he begins thinking about murdering Duncan. Macbeth says,

"If good, why do I yield to that suggestion Whose horrid image doth unfix my hair And make my seated heart knock at my ribs" (Shakespeare, 1.3.138-140).

In Act One, Scene 4, Macbeth once again displays his ambitious nature after King Duncan declares that Malcolm will be next in line to sit on the throne. Macbeth describes his ambition as "black and deep desires" by saying,

"The prince of Cumberland! That is a step On which I must fall down, or else o'erleap, For in my way it lies. Stars, hide your fires; Let not light see my black and deep desires. The eye wink at the hand, yet let that be Which the eye fears, when it is done, to see" (Shakespeare, 1.4.50-55).

Macbeth's immediate thoughts about murdering the king reveal his ambitious nature.

After Lady Macbeth reads her husband's letter, she expresses her concern that Macbeth is too kind and compassionate to carry out the murder. Like her husband, Lady Macbeth displays her ambitious nature by summoning evil spirits to make her callous and wicked enough to help assassinate King Duncan. Lady Macbeth then plans Duncan's murder, convinces her husband to kill the king in his sleep, places the bloody daggers back in Duncan's chamber, and helps Macbeth disguise his involvement. Overall, both characters are extremely ambitious and commit regicide in order to become king and queen of Scotland. As the play progresses, Macbeth continues to demonstrate his ambition by ruthlessly murdering anyone who challenges his reign.

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.