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...
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