What effect does it have on the reader if Lady Macbeth is depicted as evil?

One of Shakespeare's primary purposes in depicting Lady Macbeth as evil and malicious is to motivate the reader to have sympathy and understanding for the plight of Macbeth.  As Macbeth evolves as a character, there are two primary forces that usher him towards the evil deeds that he commits.  First, the three weird sisters plant prophecies that lure Macbeth towards his evil ways and then Lady Macbeth serves as a cunning and manipulative force in forcing Macbeth into his eventual evildoing.  For the reader, it is evident that Macbeth is coerced by Lady Macbeth and tricked by the weird sisters, which in part absolves him of the guilt that he eventually acquires from committing many murders throughout the play.  Moreover, these acts fuel Macbeth into madness and he loses all self-control and rationality, leading to his eventual ruin.  It is in seeing that these forces push Macbeth to his eventual demise that Shakespeare creates a tragic character in Macbeth.  Without these outside sinister forces the reader would not be able to see Macbeth in a tragic light and feel sadness at his downfall.