Macbeth by William Shakespeare primarily depicts the fall of Macbeth because of his own pride as well as his willingness to believe the false voices that speak into his life and move him to action.
One of those voices is his wife, Lady Macbeth. She is obviously a woman who knows what she wants, and her ambition certainly matches that of her husband. In fact, every time Macbeth wavers in his plan--or should I say his wife's plan--to kill Duncan and usurp the throne, Lady Macbeth is there to prop him up and spur him on.
She does this through various means, but her favorite method seems to be shaming him and questioning his manhood. She does this from the time he arrives until the time he finally commits the murder. She even continues this after the fact, when it becomes clear that Macbeth is suffering from guilt over his actions.
When we first meet Lady Macbeth, she is reading the letter from her husband in which he tells her about the witches' predictions and the subsequent honors given to him by the king. Nothing else. No mention of his own ambitions and desires, simply the facts as he experienced them. Her very first words after reading the letter aloud give us a pretty good idea of the Macbeths' relationship.
She says that she knows he wants to be king as badly as she wants to be queen, but she knows he is too noble and honorable (traits she obviously considers weaknesses) to take what he wants by any kind of deviousness or force. She wants her husband to come home quickly so she can begin to convince him to take action immediately--all before she learns that Duncan will be in their home that very night.
In Act I scene v, we begin to see even more frightening things about Lady Macbeth. She cries out to the spirits, asking them to take away all her feminine (human) emotions and fill her with "the stuff" it takes to do the deed she intends to do. She says:
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
The effect and it! Come to my woman’s breasts,
And take my milk for gall, you murd'ring 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
To cry “Hold, hold!”
Clearly this is an imposing and formidable woman, and Macbeth is probably right to be at least a little afraid of her.
In the next few scenes, the Macbeths go back and forth on this issue of killing Duncan. In scene vii, his wife can tell that Macbeth is wavering, and she attempts to strengthen his resolve in one of the only ways she knows how, it seems: she insults his manhood.
After her belittling tirade, Macbeth says this to her:
Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males.
The implication of his statement, of course, is that she is much too strong (dominant, unrelenting, harsh) to have female children; instead she should only have male children because her "undaunted mettle" is suitable only for males. It is an interesting comment, considering her earlier plea for the spirits to "unsex" her. Seems as if she got her wish.
Her strength does no absolve Macbeth from his actions, but she is certainly one of the driving forces behind them.