In Lady Macbeth's first speech (Act I, scene 5), she separates herself from the idea of kindness when she says to Macbeth that she fears he is "too full o'th' milk of human kindness".
Later in the same scene, she describes her exhilaration on hearing that Duncan has arrived (beginning around line 40).
...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! Make thick my blood,
Stop up th'access and passage to remorse...
Lady Macbeth is the driving force behind the murder of Duncan and she clearly defines her affiliation with evil and cruelty in these lines. She continues to use language that identifies her with evil intentions and evil iconography as she tells Macbeth to be like a serpent and suggests that if he does not murder Duncan he is a coward.