In Shakespeare's Macbeth, Lady Macbeth is evil personified at the start of the play. Her casual approach to murdering Duncan makes her seem witch-like.
Lady Macbeth is clearly comfortable with murder in planning the death of Duncan, the King of Scotland. When she hears Macbeth's report of the witches' prophecies, she is enthralled. When she realizes the King is coming to visit, she quickly decides that he will never leave the walls of their castle alive.
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. (I.v.39-41)
In Act One, scene five, Lady Macbeth does not think her husband is evil enough to become king by killing Duncan.
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(15)
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (13-18)
Lady Macbeth notes that her husband could be king, but is too kind to take a short cut (murder) to win the throne. She recognizes that he does have ambition, but not "evil" ambition. So she wishes him home quickly so she can pour her poisonous ideas into his ear so nothing stands in their way of the "golden round" (crown):
Hie thee hither,
That I may pour my spirits in thine ear,
All that impedes thee from the golden round... (22-25)
Though Lady Macbeth is villainous, she must have a soft heart. For she calls on the dark spirits to take out of her any gentle, womanly feelings so that she can do what needs to be done. First she asks that she have the capacity for cruelty:
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! (41-43)
She wants to feel no remorse for her actions that might interfere with her plan. She asks that anything nurturing about her (such as mother's milk) be soured. She asks that "the dunnest smoke of hell" cover her actions so that not even her knife can see the evil she will take part in (personifying the knife with the ability to see)—so that even Heaven will not see and stop her. (Remember that killing a king—regicide—is a mortal sin.)
When Macbeth comes home, Lady Macbeth speaks of her plan. Macbeth does not say "no," but by scene seven, he is having second thoughts. She is furious, telling him what she would do if she had promised it:
I have given suck, and know
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... (61-64)
So Macbeth agrees. After he kills the Duncan, he brings the daggers back with him. Too afraid to return them, Lady Macbeth does so, smearing the King's blood on his guards so they will be blamed—she is cold, heartless and evil.
The witches are similarly evil. Hecate is mad at the three witches because they have allowed Macbeth what he wants, but evil has not been "glorified" ("show the glory of our art"). They have not sealed his doom and his eternal damnation:
...all you have done
Hath been but for a wayward son,
Spiteful and wrathful: who, as others do,
Loves for his own ends, not for you. (10-13)
She wants to demoralize and destroy Macbeth:
...by the strength of their illusion
Shall draw him on to his confusion. (28-29)
Lady Macbeth's dedication to evil is much like the witches'.
Like a witch, she's ruthless, cunning, secretive, manipulative, and dangerous. Also very witch-like is the fact that she doesn't care about anything except fortune and power. The lives of other people mean nothing to her. Their futures and plans are like dirt underneath her shoes. She will stop at nothing to get her own way. She's so witch-like, all she's missing is a broom.
An example of her coldness and zealous thinking is when she talks to herself about her husband's future. She decides she's going to have to nag him hard and long, in order to make sure he doesn't mess things up by being too indecisive or causing some other obstacle.
Another example is when she literally calls upon the demons of the underworld to fill her with evil. She wants to shed everything about herself that's even remotely feminine or emotional. Again, the reason is that she doesn't want second thoughts, moral dilemma, or any other such problem to stand in the way of their plans.