Lady Macbeth persuades Macbeth to kill Duncan so that he can become king. She says to Macbeth that he's a coward because at first, he doesn't like the plan to kill Duncan, but in reality, she's the coward.  Lady Macbeth wanted Duncan dead, but she wouldn't actually kill him herself, she gets Macbeth to do it.

Act 1, Scene 5
" ...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 the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
Th'effect and it! come to my woman's breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances
You wait on nature's mischief! come, thick night,
And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes,
Nor heaven peep through the blanket of the dark,"