Lady Macbeth is one of Shakespeare's strongest women and is often considered a villain of the play. In an odd way, that's high praise, as this important function was usually reserved for a male character in the play.
We first meet her in Act I, scene V. She is reading a letter from Macbeth, in which he describes his prophetic meeting with the witches. "Hail, king thou shalt be!" is all the motivation that Lady M needs to decide that her husband should become king sooner rather than later:
Glamis thou art, and Cawdor, and shalt be
What thou art promised: yet I do fear thy nature;
It is too full o' the milk of human kindness...
And, in recognizing her husband's weakness, a kind heart, Lady M. sets herself up as the driving force behind a plot to kill the present king, Duncan, while he sleeps in their home that night.
In her famous lines (39-55), she invokes whatever magic it might take to give her the strength and courage of a man (implying that her husband lacks this) to get the job done -- which is murder.
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!
She sets herself up as a villain here, invoking the forces of Hell and cautioning those of Heaven to stay out of her scheme.
Macbeth arrives and she not only greets him by the titles he owns (Glamis and Cawdor), but also calls him "Greater than both hereafter." She wastes no time in telling Macbeth that Duncan will never leave their home alive ("O, never/Shall sun that morrow see!"). While Macbeth isn't convinced of her hastily described plan, Lady M, is clearly in charge -- "Leave all the rest to me."
In Act 1 Scene 5 of Macbeth, Lady Macbeth receives a letter from her husband giving her details about the witches' prophecy. Lady Macbeth is excited by the promises that have been made to her husband, but she then begins to doubt whether or not Macbeth will be able to do all that he has to do in order to receive his new titles. She says, "Yet do I fear thy nature: It is too full o'th'milk of human kindness, to catch the nearest way. Thou wouldst be great; art not without ambition, but without the illness should attend it" (I.v.16-20). Lady Macbeth thinks that her husband has too much good in him to ever play false or to use aggression to achieve his goals. At this point, Lady Macbeth resolves to help her husband achieve his new titles. She says, "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!" (I.v.40-43). Lady Macbeth no longer wants to remain passive; she instead wants to herself to be full of aggression so that she can convince Macbeth to do whatever it takes to achieve that which has been promised by the witches.