The doomed Lord and Lady Macbeth are not only partners in matrimony but partners in crime as well. The couple share the same desire, which is to achieve the highest honor in Scotland and become King and Queen. However, it becomes apparent early in the play, that there is somewhat of a reticence in Lord Macbeth to achieve his ideal by the shortest way, which means that he has to commit an unholy deed in order to achieve what he desires. Lady Macbeth, though, comes across as much more ruthless than her husband as is made clear in her musings after she had received a letter from him stating his success in having been awarded the new title, thane of Cawdor:
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; 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.' 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 is afraid that Macbeth is too kind to do anything malicious to achieve his ambition. She implies that he is caught between desire and morality. Although he wants to be king, he does not possess the malice to do something illegal and would rather take the morally just route. Her ruthless ambition is indicated by her need to have him rush home so that she can, with words of courage, inspire him to do everything that is necessary for him to become king, for that is what fate and the witches have promised.
In this sense, therefore, Lady Macbeth is her husband's spur. She is the one who awakens in him the malice that is required to achieve the crown in the quickest way possible. The only way to do this would be to kill Duncan and of course, his sons, for they are nearest in blood to him. Added to that, the king has already named his son, Malcolm, heir to the throne by naming him Prince of Cumberland. Since Macbeth is next in line after Malcolm and Donalbain, he would have to get rid of them as well.
Lady Macbeth has prepared herself for committing the foulest of deeds, the betrayal of her king through assassination. King Duncan would be visiting their castle and she sees it as an ideal opportunity for her and her husband to perform their mischief. She has called on the spirits of darkness to envelop her in evil so that she can commit this malice.
...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! make thick my blood;
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances
You wait on nature's mischief! Come, thick night,
And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes,
Nor heaven peep through the blanket of the dark,
To cry 'Hold, hold!'
When Macbeth arrives home, she is ready for him and almost immediately starts urging him on. She tells him to present a visage of friendship and conviviality so that there is no suspicion on him. She tells him that Duncan 'must be provided for' - a clear suggestion that he has to be killed.
Lady Macbeth is later very critical of her husband when he expresses doubt about continuing with their plot. She questions his courage and consistently mocks him and nags him. She lays down terms and tells her husband that she would measure his love by what he proceeds to do thereafter. Macbeth is in a quandary but he later accedes to his wife's insistent demands after she guarantees that their plot will not fail.
Lady Macbeth provides Duncan's guards with a sleeping potion and Macbeth murders him in his bed. His wife arranges the evidence in such a way that the guards would be implicated in the king's murder. When the murder is discovered, Macbeth slaughters the sleeping guards, apparently out of loyalty and grief at his golden liege's untimely and vile murder. When Malcolm and Donalbain hear the tragic news they flee, fearing for their lives. This makes it easier for Macbeth to claim the throne since their escape makes the two heirs suspects and they cannot, therefore, be considered.
And so begins Macbeth's murderous reign. Lady Macbeth continues to support him and criticizes him when he imagines seeing Banquo's ghost after he has had him murdered. When he turns pale and starts jabbering at the ghost during a banquet, Lady Macbeth comes to his defense, saying that his condition is caused by a childhood illness that he is afflicted with from time to time. She scolds her husband for showing such foolish weakness.
It is clearly his wife's support that encourages Macbeth. He becomes paranoid about his safety and sends assassins through the country to execute whoever he deems a risk. His tyranny envelops Scotland like a disease and causes much pain and suffering. His malice knows no bounds for he even has Macduff's entire family wiped out.
Ultimately, Macbeth is haunted by images and dreams of his brutality and he cannot sleep. Lady Macbeth, likewise, suffers. She later loses her sanity and eventually commits suicide. Macbeth is later killed by Macduff, bringing an end to his reign of terror.