After Lady Macbeth reads the letter, in which she learns of Macbeth's meeting with the weird sisters, what they promised him, and how one of their predictions had already come true, she tells us much about her opinion of Macbeth as well as a lot about herself. From her first words, we can see that she has already decided that, one way or another, what the sisters predicted will come true ("Glamis thou art, and Cawdor; and shalt be / What thou art promised"). Further, she's already decided how it will happen, without consulting her husband. We see immediately that she's the leader of this duo, even though Macbeth is the king-to-be.
Then she tells us what she knows of Macbeth: She is afraid that his law-abiding and maybe even pious nature will stop him from murdering Duncan (which she has already decided will happen). He is "too full o' the milk of human kindness / To catch the nearest way" meaning that he is too compassionate to murder Duncan, which is the easiest and fastest way to become king himself.
She knows he is ambitious and wants to be great, but he lacks "the illness should attend it." This line is telling, as she clearly believes that all powerful people are crooked to some extent (rather like a lot of people believe all politicians are crooked--apparently, this sentiment is as old as history itself). This line also helps her justify what she has planned (if all kings are crooked, then what's the big deal in playing dirty to become one yourself?).
She goes on to say that whatever her husband would do to become powerful is limited to what he can do "holily," or what he can reasonably get penance for. He would not "play false"--pretend or cheat--and yet...he would not be opposed to winning (the crown) even if the way he got it wasn't completely fair. He is, in short, the sort of man who knows what needs to be done to achieve his goals ("Thus thou must do, if thou have it") but at the same time is prone to stop before he does it rather than doing it first, then repenting ("That which rather thou dost fear to do / Than wishest should be undone.") In other words, Lady Macbeth is of the Rear Admiral Grace Hopper school of thought (It is often easier to ask for forgiveness than to ask for permission), but she knows Macbeth is the more cautious sort.
She plans to "help" him by convincing him, to "chastise with the valour of [her] tongue" the parts of his nature that will keep him from doing it her way--the wicked way. In other words, she plans attack all that is good and just and admirable in her own husband so he will commit regicide.
The weird sisters get a lot of flack in this play, but it's Lady Macbeth who is the devil.