Your question presumes that Shakespeare portrays Lady Macbeth as evil and cunning; while she does do some pretty awful things, I'd characterize her as ambitious and ruthless. In either case, Shakespeare is clear in his portrayal of her, which we can see by examining her behavior throughout the course of the play.
The first time we meet her, she is reading a letter from her husband telling her of the witches' promising predictions for his future. There is, apparently, love between them; Macbeth wants to share his fortuitous news with the woman he loves. She undoubtedly loves him, too; however, she seems to know him well enough to be nervous for their future.
The first words she speaks after finishing the letter paint her husband--a man great in valor and unafraid of battle--as a man lacking in ambition. That's not true, as we find out later, but he clearly has less ambition than his wife. She's afraid he won't be willing to do what needs to be done (presumably murder) to achieve the goal (becoming King of Scotland). Once he arrives in person, she begins her campaign to spur her husband's ambition into action.
On the night of the Duncan's murder, Lady Macbeth has to re-convince her husband to do the deed. She is ruthless in her ambition, insulting his manhood and impugning his courage, saying she will do it if he is unwilling or unable. When it comes down to it, though, she can't commit this cold-blooded murder--the one thing which saves her from being evil in my book.
It's true that she is perfectly willing to goad her husband to commit murder; it's true she dispassionately both sets up and stages the crime scene; and it's true she is right behind Macbeth in these evil deeds, prodding him to action and telling him a little water will wash away their guilt.
Soon after, though, Macbeth must sense some softening in his wife, for he fails to confide his next murderous plans to her. He plans and executes the murder of Banquo and the assassination of MacDuff and his family without telling her. His resolve grows as hers, apparently wanes.
In her most famous scene, she finally succumbs to her guilty conscience, unsuccessfully attempting wash her hands clean of Duncan's blood. Eventually Lady Macbeth takes her own life, apparently from that same sense of guilt. It strikes me that an evil, cunning person would probably not have had any compunctions about her actions.
While Lady Macbeth is ruthless in her ambition and does achieve the Crown, she is unable to enjoy it because she is not as evil as some of her actions suggest. She is racked with guilt and dies separated, at least emotionally, from the husband she was a partner to at one time. Lady Macbeth is not a sympathetic character; however, she may not be an evil one, either.