Often our sympathy is measured by how well we know the character and what we know of him/her. Until Lady Macbeth is beleaguered by guilt, all we know about her is her desire to do something evil, and that she calls upon "the spirits" to give her the courage to do that evil. She feels guilt, yes, and wishes she could undo her deed, but one might argue giving into guilt rather than seeking redemption for what she has done is a less moral closure to all of her actions. Think of people who have done dreadful acts and lose their mind afterward because of guilt: do you have sympathy for them? Pity seems a bit removed from sympathy; it requires less connection to the person, less empathy. Perhaps feeling pity for Lady Macbeth would be a bit easier for me than feeling sympathy for her. She lost my good will when she calls on the evil spirits in 1.5.40-55, asking them to make her less of a woman so that she can convince her husband to kill Duncan.
Personally, I do feel some sympathy for Lady Macbeth. She, at least, has realized her sins. She is tormented by her wrong-doing, and understands that what has been done can never be undone. She cannot bring Banquo back, she cannot erase her part in the murder.
You could definitely play up the other side of it, however. She knew full well what the results of her collusion with her husband would be. Greed was her motivation, so how sorry can we feel for someone who is tormented in life by the deliberate death of another?
By the way, at the link below you can view a modern translation of this scene (and all of Macbeth) side-by-side with the original text.
A bit of both perhaps? Honestly, Lady Macbeths ruthless and absolutely dismissive nature, set in the beginning of the play, elivits a lot of negative feelings for her in the minds of the audience. But we also realsie that she was an excellent wife, caring and undertsnaidng of her husband's flaws and very often, willing to put he rown life on hold and on risk, for his betterment and success (ex: the time she volunteered to put the daggers near Duncan's chamberlains who were later accused of murderng the former intheir 'drunken stupor'). She was completely and soemtimes, unreasonbly committed to her husband's upliftment and would always help him in all his actions.
For this, one feels a little sympathy for the woman. However, killing anyone, no matter whom, has no excuse. And for that one is to be punsihed. Assassinating Duncan, a virtuous leader, a man aid to be blessed and graced By God, was unjustifiable and the husband-wife duo deserved reprimand for it.
Another occassion on which one feels sympathy for the woman, is the time she was caught sleepwalikng by a gentlewoman and a concerned doctor. The abrupt, short words used by her and references she made to Banquo's ghost (that was seen by her husband at the first feast after his coronation), Duncan's rich blood, hell being murky and the guilt they bore in their hearts is heart rendering. Also her constant action of trying to wipe off the lood stained on her hands, elicits some sympathy for the woman.