Lady Macbeth is Shakespeare's most evil feminine creation. Her satanic prayer to the forces of darkness in Act 1 is chilling to modern readers and it would have been absolutely terrifying to Jacobean groundlings watching the horror unfold in Shakespeare's own Globe Theatre. Most critical analysis of Lady Macbeth focuses on her as catalyst for Macbeth's first murder, that of Duncan, and the linear progression of her deteriorating mental state, culminating in her sleepwalking scene. 

However, the most interesting facet of Lady Macbeth's character is hardly ever explored: that she herself intends to commit the murder of Duncan, while her husband merely plays the smiling host. This precious detail gives Lady Macbeth's invocation new weight and her character new depth. John Dover Wilson, the editor of the first edition of The Cambridge Macbeth, was one of the first scholars to bring this hypothesis to light. As he writes in his introduction to the play:

The whole point of Lady Macbeth's invocation is that she intends to murder Duncan herself. She speaks of 'my knife' and of 'my fell purpose.' And the same resolve is implied in everything she says to Macbeth after his entry. She bids him put "This night's great business into my dispatch"...she tells him he need do nothing but look the innocent and kindly host; she dismisses him with the words 'Leave all the rest to me'. All this seems obvious directly it is pointed out, though once again no one appears to have noticed it before, simply because in the end the murder is of course performed by Macbeth himself; and must be, however the drama is shaped. I suggest, by means of a further dialogue between husband and wife, preceded perhaps by a scene in which, going to the bedroom knife in hand, she cannot bring herself to the action; and I further suggest that when he reached this point in 1606 Shakespeare found he had no room for such developments and had to extricate himself as best he could. And how triumphantly he does it! First he writes a soliloquy ('If it were done, when 'tis done') for the beginning of scene 1.7, which conveys the impression that Macbeth was intending all along to do the deed himself; he then later in the same scene makes the guilty pair talk as if they were proposing to do it together; and finally, though he sends Macbeth to the bedroom alone, he brings Lady Macbeth on to inform us that she has already been there, and that 

Had he not resembled
My father as he slept, I had done't.