I do agree with the statement that Lady Macbeth is one of the most evil women in literature.
Postings of direct quotes are shown above, so I will simply summarize. Act II, scene ii, is seen by some people as a turning point. I believe it may be one, but not the only one.
In the relationship between Macbeth and Lady Macbeth, there is a shift here in who has the upper hand in the relationship. Macbeth is a true warrior, who has proven himself in battle. However, when he murders Duncan, his king, friend and cousin, I think this goes completely against the man's character. He really cares for Duncan. He blames his vaulting ambition for pushing him to complete the act, but also has a nagging and ambitious wife at home who is unrelenting and insulting.
After the murder, Macbeth starts to lose his grip. He begins babbling about the king's guards praying and his inability to ask for a blessing. He leaves the scene of the murder with the incriminating daggers. He frets and worries, and even refuses to put the daggers back next to the sleeping guards.
Lady Macbeth proves what she is made of here. First she tries to get him to snap out of it, telling him not to worry. Then she begins to insult him. When she finally takes the daggers back, she returns to tell him what a coward he is. While Macbeth laments that no amount of knocking can wake Duncan now, Lady Macbeth tells him that washing their hands and acting innocent is as good as cleansing them of the sin they have committed. (And the Elizabethans believed it WAS a sin to kill a king.) While this doesn't connect with Macbeth, Lady Macbeth believes all will be well. She will be queen and no one will raise a voice against them.
It is Lady Macbeth's ambitions that push the murder to its fruition. Had she backed off when Macbeth originally voiced his doubts over the plan, it would have stopped. However, this woman is conniving and evil, and pushing her husband to go against what he knows is morally correct shows how little regard she has for anyone but herself. Ironically, her casual acceptance of the situation is only temporary: it ultimately will drive her insane.
Lady Macbeth is the most evil woman in literature because she broke the cardinal rule of hospitality. She instigates a treasonous murder of guests in her own house. She calls her husband a coward for not making the murder appear to be a mutiny by the groomsmen, and she returns to the bed chamber to place the daggers by the grooms and smear their faces with his blood.
Infirm of purpose!
Give me the daggers. The sleeping and the dead
Are but as pictures; ’tis the eye of childhood
That fears a painted devil. If he do bleed,(70)
I'll gild the faces of the grooms withal,
For it must seem their guilt.(II. 2)
Lady Macbeth appears to be an "unnatural" woman in that she stated in Act I that she would kill a baby at her breast in order to fulfill a vow.
I have given suck, and know(60)
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this.(65) (I,7)
It is this tendency toward brutality in order to gain power that makes Lady Macbeth to be the most evil woman that she is portrayed as in literature. She does not appear to have the normal tendency to protect life that is found in all women. She is not abhorred by murder in her own house, but she protects and defends it to ensure her husband's rise to the throne. However, in the end, her humanity returns, and she is guilt stricken by a sickness of the mind. Eventually, she commits suicide. A death in Elizabethan times that condemned the soul to eternal hell.
There is a sense of evolution through this scene that illustrates Lady Macbeth's progress from one of fear to one of resolution. At the beginning of the scene, she is worried that the deed has not been done:
Alack, I am afraid they have awaked,
And 'tis not done. The attempt and not the deed
Confounds us. Hark! I laid their daggers ready;
He could not miss 'em. Had he not resembled
My father as he slept, I had done't.
There is a worry on her part, about what it is that they have planned, but she does say at the end that she would have done it, which hints at the idea that she doesn't trust her husband to follow through with the murderous power-grab.
The great transition to resolution comes in the line when she discovers that Macbeth has brought the knives out of the room to her:
Why, worthy thane,
You do unbend your noble strength, to think
So brainsickly of things. Go get some water,
And wash this filthy witness from your hand.
Why did you bring these daggers from the place?
They must lie there: go carry them; and smear
The sleepy grooms with blood.
At their sight, she becomes irritated and says Macbeth is,
Infirm of purpose!
Give me the daggers: the sleeping and the dead
Are but as pictures: 'tis the eye of childhood
That fears a painted devil. If he do bleed,
I'll gild the faces of the grooms withal;
For it must seem their guilt.
She calls him infirm and a child. At this point, at the end of the scene, Lady Macbeth has switched roles with her husband. He is the guilty, fearful one, and she is the heartless, greedy "Queen" who will stop at nothing to keep her newfound crown.