In Shakespeare's Macbeth, Lady Macbeth uses Macbeth as a tool to get what she wants.
Lady Macbeth is very excited to learn that her husband may be king. If this happens, she will be queen. She frighteningly manipulates her husband to murder Duncan. It is ironic that Macbeth seems strong enough on the battlefield to clear a path before him on foot with nothing but he cannot stand up to his wife. However, Lady Macbeth's words have an enormous effect on her husband.
Lady Macbeth uses Macbeth's ego to push him in the right direction to commit murder. She tells him that when he said he was willing to murder, he was acting like a man—he was brave. Now she questions his manhood by accusing him of being fearful and trying to back out.
Macbeth roars at his wife to be quiet: he declares that no one is braver than he is.
I dare do all that may become a man;
Who dares do more is none. (I.vii.50-52)
Unconvinced (or at least acting that way), she points out that had she promised to do so, she would willingly kill a baby nursing at her breast.
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. (60-65)
Lady Macbeth uses Macbeth's ego against him to manipulate him to agree to murder Duncan: friend, cousin, king and guest.