Despite Lady Macbeth's early bravado concerning Duncan's murder, the way she shames her husband for his reluctance to commit the murder, and her belittling of him when he initially regrets it, there are some clues that she is not as steel-nerved and ruthless as she'd like Macbeth to think. When she first learns of the Weird Sisters' remarks to her husband, she prays to be "unsex[ed]" and filled "from the crown to the toe top-full / Of direst cruelty" (1.5.48,49-50). Further, she says,
Make thick my blood.
Stop up th' access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
Th' effect and it. (1.5.50-54)
She prays to have any quality associated with being feminine removed from her body: she doesn't want to be soft or gentle and kind. She wants to be hard and terrible and cruel. She asks that any feelings of remorse or compassion be quelled so that she is able to go forward with what she plans. Why would Lady Macbeth need to pray for these capabilities if she already felt herself to be cruel and ruthless? She must feel that some remorse or compassion could flare up inconveniently and make her regret her decision or feel guilty about it. In other words, she is not, by nature, evil.
Later, while Macbeth is away, performing the murder, Lady Macbeth says that "Had [Duncan] not resembled / [Her] father as he slept," she would have killed the king herself (2.2.16-17). This implies that she has a certain sentimentality, despite her hopes that she will be cruel enough to perform the murder herself; when the chips are down, she cannot do it. Her "better" feelings seem to overrule the worse when it matters most.
Ultimately, then, when Lady Macbeth sleepwalks and talks, she reveals that those better feelings have not gone away. They have persisted, and her guilt about Duncan's murder—and all of the other murderous acts her husband has performed since—has grown and festered inside her until it has found some release. She can hide it all while she's awake, but she, while she sleeps, those true feelings come out. We see that Lady Macbeth was destroyed by her ambition: she manipulated her husband for personal gain, and he became a monster, and now her conscience cannot support what she has done—and Shakespeare gave us clues about her true character from the beginning.