Ah, great question. Thinking about this lets us dig into the heart of the play.
We feel both emotions for Lady Macbeth because of the different strains the situation puts on her, and because of the way the different sides of her character come into play.
On the one hand, she is loyal to her husband, and wants him to be king. What could be better?
On the other hand, she drives him to kill when his conscience would weaken him.
On one hand, she knows her husband really well, recognizing that his character isn't harsh enough for the challenges this situation puts on him. She strengthens him.
On the other hand, again, she drives him to kill.
On one hand, she believes the information from the witches immediately.
On the other, she casts aside her own loyalty.
On the one hand, she clearly loves her husband, and is clearly wracked by guilt over what she's done... and on the other, she is so harsh that she says she'd kill a baby if need be, to get the job done.
I sympathize with her... but she also disgusts me.