Lady Macbeth could also be considered to be the antagonist or villain of the play, which means that she is operating in opposition to Macbeth, or against his best interests. Shakespeare makes it pretty clear in her speech in Act One, scene five, that she considers Macbeth too kind-hearted to do what must be done for him to become king. She then calls on the agents of darkness (evil). She incants:
...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.
She has just, in effect, created a pact with the devil, and certainly in the crucial early scenes, as Macbeth is choosing his course of action, her instigation is a key motivation to his choices. So, Lady Macbeth can be seen to be the antagonist and evil influence that prompts Macbeth towards his tragic course of action.
In Macbeth, Lady Macbeth serves as a symbol for Macbeth's inner conscience. At the beginning of the play, she is the one who challenges his moral conscience as Macbeth debates whether or not he should murder King Duncan to take his title. At first, Macbeth is resolved to let Fate take its toll, especially since he has received the title Thane of Cawdor just as the witches have predicted. However, Lady Macbeth is the symbol of Macbeth's darker side, and she pokes fun at his sense of manhood to encourage him to go after his ambitions. Of course, he follows her suggestions. In the end, Lady Macbeth sees that all her actions have come to no good ends, and she feels guilty over the death of Lady Macduff and her children. Shortly after, Macbeth also admits that his evil deeds have brought no good.