According to literary criticisms of Shakespeare's work, Lady Macbeth is one of the most evil female characters he wrote. She is manipulative, cunning and more ambitious than Macbeth in the beginning of the play.
After she receives her husband's letter detailing the witches prophecy which predicts that Macbeth will be king, she immediately begins to plan Duncan's murder.
Lady Macbeth, in Act I Scene V, as she reads her husband's letter, she decides that Macbeth:
Lady Macbeth: ...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. (I.v)
Deciding that she will instill in him the courage and strength that he will need to kill the king. She challenges his masculinity, which in the beginning of the play, she defines as aggressive and violent. Lady Macbeth's role is co-conspirator, accessory to murder, all motivated by unchecked ambition and a lust for power.
Lady Macbeth hungers for power. In Act 1, scene 5, when we first see her and she gets the letter from her husband telling her of the witches' prophecies, she immediately says that he shall be "what thou art promised": "Glamis thou art, and Cawdor; and shalt be / What thou art promised:..." (1.5). She goes on to say that he has ambition, which is good, but he also has the "illness" along with it, namely, a ruthless desire to achieve at any cost. This implies that she does have that ruthlessness in her. In a few more lines, she says that she will try to persuade him to do what must be done (kill Duncan). Later in the scene, when she finds out that Duncan is coming there to visit that evening, she immediately decides that he should be killed that night. She doesn't waver and she invokes the powers of darkness to help her succeed. Raw, unrestricted ambition is her motivation.
Although I agree that Lady Macbeth's main motivation is her lust for power and potent ambition, it could be argued that her motivation simply is her undying love for Macbeth.
She is depicted as an incredibly evil character right from the start of the play, her introduction is her reading the letter and plotting a plan to kill the King for Macbeth to become King. She is ruthless and conniving but she does all of this for Macbeth. You don't hear her rambling about the way her life as Queen would be wonderful. The fact that she would risk her life, her life in the after life (as in those times they would be heavily religious and superstitious) and in the end her sanity shows the utter loyalty of love she has for her husband.