If the main theme of Macbeth is ambition, whose ambition is the driving force of the play, Macbeth's, Lady Macbeth's or both?
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.