Even though Lady Macbeth prods her husband on, she has no actual power. Her only power is in pushing him. This is why she pushes him. She wants to have the power of being queen. Malcolm calls her the “fiend-like queen,” but in reality Lady Macbeth has little to do with Macbeth’s kingdom. She helped with the initial murder of Duncan, and encouraged him to kill Duncan, but after that she was not involved in the other murders.
Lady Macbeth is inspired when she finds out about the witches’ prophecies that Macbeth is going to be king. She knows that her husband wants this. However, she worries that her husband is too wishy-washy to act on his ambitions.
Glamis thou art, and Cawdor; and shalt be
What thou art promised: 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 … (Act 1, Scene 5)
Later, Lady Macbeth does plan everything for her husband and chide him when he doesn’t follow the plan to the letter. She seems to know what she is doing in planning murders. However, she doesn’t carry out the murder herself. She needs him.
Alack, I am afraid they have awaked,
And 'tis not done. The attempt and not the deed
Confounds us. Hark! I laid their daggers ready;
He could not miss 'em. Had he not resembled
My father as he slept, I had done't. (Act 2, Scene 2)
Relying on her husband must have been hard for such a strong woman. She turns out to be not as bloodthirsty as she seemed. Lady Macbeth starts to crack. Macbeth, it could be argued, was cracking long before he killed Duncan. He was already envisioning floating daggers. By the time Macbeth has Banquo killed and sees his ghost at the banquet, Lady Macbeth is making excuses for him. It won’t be long before she loses her tenuous grasp on reality and is trying to wash her hands of blood long after it is no longer there.