Lady Macbeth says that her husband is soft and she worries that he cannot do what is necessary to become king.
When Lady Macbeth gets the letter from her husband describing his interaction with the witches, she is thrilled with the predications that he is going to get a promotion and then be king. However, she does not believe that he has what it takes to make it happen.
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)
Basically, Lady Macbeth thinks that her husband is weak, and that if there is violence to be done, he will not have the guts to do it. When she says that he is too full of “the milk of human kindness,” she is saying that he is too gentle to do what needs to be done. He is too nice a person. She implies that he needs her to “chastise” him “with the valour of [her] tongue” and guide him.
Is this the Macbeth we know? Lady Macbeth paints the portrait of a gentle man, but we have been told that Macbeth is a valiant and ruthless soldier. The bloody sergeant describes Macbeth’s deeds in battle, and tells of how he defeated the Thane of Cawdor valiantly with no thought of himself.
For brave Macbeth--well he deserves that name--
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valour's minion carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements. (Act 1, Scene 2)
In this description, Macbeth cuts a bloody passage through the battle, killing everyone in his path, and then slices the guy in half. It certainly seems like he had no problem either with violence, knowing what to do, or doing what had to be done. Lady Macbeth was nowhere to be found on that battlefield, and Macbeth was brave enough.
Why then, does Lady Macbeth describe her husband as weak? Could it be that he is just principled? He knows that killing Duncan is wrong. Even as she tries to talk him into it, Macbeth will suffer a crisis of conscience because of the fact that Duncan does not deserve to die and Macbeth does not deserve the crown. To be king would never have occurred to him until the witches put the thought in his head. Then, of course, he demonstrates sound ambition. His wife does not believe he is ambitious? She should have heard his soliloquy when he learned that Malcolm was chosen as heir to the crown instead of him.
Clearly, based on this outburst, Macbeth is ambitious and moody. He certainly seems to believe at this moment that he deserves to be king, and he is very upset when the honor is not granted to him. Malcolm is the natural heir to the throne, but Macbeth does not see it that way. He wants what he sees coming to him. The witches painted a future of glory, and now that is all he can think about. However, later, when he is back in his own castle with a cooler head, he has second thoughts.
It often does seem as if we have two Macbeths. There is the valiant and ambitious Macbeth who can plow his way through a battlefield and order the murder of his friends in cold blood, and there is the brooding and cowering Macbeth who takes orders from his wife and finds himself wracked with uncertainty and guilt. Macbeth is manipulated throughout the play at almost every turn—by either the witches or his wife. It is no wonder that he shows signs of instability.
This is a very interesting question, because to some extent this scene gives us an impression of Macbeth that does not necessarily fit in with what we have been told about him before. If we look at this scene carefully, we can see that Lady Macbeth, having been told about the prophecy of the witches is concerned above all that Macbeth doesn't have what it takes to make the prophecy happen for him. Note what she says:
Yet do I fear thy nature:
It is too full o'th'milk of human kindness,
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it.
She argues that Macbeth is too compassionate and not evil enough to truly act on the sickness of ambition to its full extent and murder Duncan. What is interesting about this is that it seems to stand in contrast from the impression of Macbeth that we were given by the Captain in Act I scene 2, and how Macbeth is presented as a ruthless, bloody and violent soldier. Remember how he is refered to as "Belladonna's bridegroom" and we are told how he "unseam'd" his enemy "from the nave to th'chops." This does not necessarily fit in with Macbeth being "too full o'th'milk of human kindness." However, Act I scene 3 makes clear that Macbeth is not free from moral scruples, as his reaction to the thought of killing Duncan shows. This points towards a somewhat ambiguous impression of Macbeth as a character.