Lady Macbeth is the most evil character in Macbeth
Throughout the play the character of Lady Macbeth is consistently displayed as the most evil character in the play through her ambition, cruelty, and manipulation.
 

AMBITION

- When Lady Macbeth leans about the witches' prophecy that Macbeth will ultimately be King, she fears he will be too kind-hearted to achieve this status so she begins to take matters into her own hands. Her soliloquy imploring dark powers to take all compassion from her is appalling in its unnaturalness:

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 is far more ambitious then Macbeth is. She appears to be mentally strong and her conscience did not seem troubled by the murders that she was a part of. Her determined ambition is what keeps Macbeth focused on committing the murders. It is not until later in the play when Macbeth is able to adopt ambition of his own as he plans the murder of Macduff's family on his own. However, this ambition he receives is that which came from Lady Macbeth.

CRUELTY

Lady Macbeth rarely shows any sympathy towards any characters throughout the time of the play. When she finds out the prophecies given to Macbeth by the witches, she immediately takes action and creates a plan. She works out the details of the plan to kill the king, and when Macbeth cannot return the blood-stained daggers to Duncan's room, she takes command of the situation and returns the daggers herself.

Infirm of purpose!
Give me the daggers. (II, ii, 51-52)

She also plays a key role in the murder of Banquo. During the feast whilst there are guests over, Macbeth is so unsettled by seeing the ghost of Banquo that he nearly has a mental breakdown. Lady Macbeth shows no regard for this as she immediately ridicules him. Lady Macbeth's true cruelty is shown when her death has no effect on the play as she is not considered a hero. Her character is that of such pure evil that it is difficult for the audience to sympathize for her on a personal level. Her crimes were considered so brutal that it would be difficult for anyone to ever forgive her.

MANIPULATION

Lady Macbeth is an exceedingly conceded character as seen in as she openly defies heaven. Her true nature can be reflected through the imagery of blood, violence, death, as from the beginning of the play she is set out on an inevitable course of destruction. Macbeth is first introduced as an admirable individual and a hero to Scotland, it is Macbeth's fatal ambition that is unleashed by Lady Macbeth. His true personality can be revealed as he admits:

I am in blood

Stepped in so far that should I wade no more,
Returning were as tedious as go o'er (III, iv, 136-138)
Lady Macbeth instigated the actions which eventually led to Macbeth committing the acts that he did. Throughout the play the audience is constantly reminded of Macbeth insecurity as he is always questioning his own actions. Lady Macbeth's primary role in the play is to give Macbeth the vital push and then sustain him until he can control his own conscience and actions. Lady Macbeth uses a specific techniques to ensure that Macbeth keeps to his purpose such as accusing Macbeth of being a coward. As Lady Macbeth herself possesses characteristics that are regularly only present in a male such as single-minded courage and cruelty. She shuns Macbeth for his failure to live up to the standard which she, as a women, has set. Using her physical characteristics of a women, and mental characteristics of a man, Lady Macbeth ridicules her husband for not being able to murder Duncan as seen in the following quote:

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 plucked my nipple from his boneless gums,
And dashed the brains out, had I so sworn as you
Have done to this (I, vii, 54-59)