Lady Macbeth is portrayed early in the play as a very strong character, though she is also unquestionably evil. One important quote that demonstrates her character and her influence over her husband is found in Act I, Scene 4, when she says that she fears her husband lacks the ruthlessness necessary to fulfill the destiny the witches have laid out for him:
Yet do I fear thy nature;
It is too full o’ the milk of human kindness
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. What thou wouldst highly,
That wouldst thou holily; wouldst not play false,
And yet wouldst wrongly win.
In other words, she fears that Macbeth, while ambitious, will not be willing to do what is necessary to realize his ambitions. So in a chilling passage later in the scene, she resolves to push her husband toward his destiny:
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!
Later in the play, as Macbeth agonizes over whether or not to murder Duncan, his wife challenges his masculinity, claiming that he has sworn to fulfill his destiny, and that no real man would go back on a solemn oath:
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, had I so sworn as you
Have done to this.
She even suggests that her love for him depends on his carrying out the murder of Duncan:
From this time
Such I account thy love. Art thou afeard
To be the same in thine own act and valor
As thou art in desire? Wouldst thou have that
Which thou esteem'st the ornament of life
And live a coward in thine own esteem...
Later in the play, Lady Macbeth is a broken woman. Macbeth's actions, and his ambition, eventually spiral out of her control, and she last appears as torn by remorse, in the famous "Out! damn'd spot!" scene. But in Act I, Lady Macbeth shows herself to be committed and ruthless, and intent on driving her husband toward what she believes is their shared destiny. Macbeth realizes the implications of the witches' prophecy almost immediately, and his wife's influence is instrumental in pushing him to overcome any qualms that he has.