behind Macbeth's downfall
Lady Macbeth? The driving force behind Macbeth's downfall? Certainly not. Macbeth was completely and solely responsible for all the acts of great evil which were to lead to his downfall, and to even suggest the blame can be shifted on his wife is ludicrous.
From his very first meeting with the witches, Macbeth's mind became instantly plagued with thoughts of murder and treachery. The guilty start that Banquo noticed:
"Good sir, why do you start, and seem to fear
Things that do sound so fair? "
showed us that the thought of murder was already at the back of his mind. This showed us that Macbeth could not have been as honourable and trustworthy as people believed him to be, given that if he had had but a shred of integrity, murder would have been the last thing on his mind. The witches cannot corrupt the virtuous (like Banquo), they can work only on the evil that they already find in their victim's mind. At this point, Macbeth (and everyone else), was not aware of this evil inside of him, which is why he was so horrified by the hideous imaginings that spring to mind. He was afraid of speaking of his "black and deep desires" openly, even to himself. For this reason, he sends a letter to his wife, explaining the situation, hoping that the thought of murder would cross her mind, and he won't have to be the one to bring it up. On receiving the letter, Lady Macbeth's first thought (as Macbeth had hoped it would be) was one of murder. She was just as ambitious, if not more so, than her husband, so much so that she would do anything, even conspire to commit murder, to get what she wanted in the end. However, she was not an evil woman, which is why she felt the need to call on the powers of darkness to aid her in what she was about to do:
" Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe...