Lady Macbeth

(I did say that she deserved her own post, right?)

The correct adjective for Lady Macbeth is 'bloodthirsty'.  We know very little of her from before the story.  She is introduced reading a letter from her husband.  In it he tells her of the three weird sisters and there prediction that he will become king.  He then tells her that he wanted to share this with her, his partner, so that she could rejoice in promised greatness.
Her reaction?  Why then, we must kill the king!  No seriously.  She spends a bit of time worrying that Macbeth isn't vicious enough to attain his ambition.  Then she learns that the king will be staying at their castle and says:

     The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. Come, you spirits
that tend on mortal thoughts, unsex me here,
And fill me, from the crown to the toe, top-full
Of direst cruelty! Mack thick my blood,
Stop up th' access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
Th' effect and it! Come to my woman's breasts,
And take my milk for gall, you murd'ring ministers,
Wherever in your sightless substances
You wait on nature's mischief! Come, thick night,
And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes,
Nor heaven peep through the blanket of the dark,
to cry  "Hold, hold!"

Just a lovely woman.  And remember, this is before she has had any chance whatsoever to talk with her husband.  She doesn't know if he has been put in the line for the throne.  She doesn't know if the weird sisters told him anything else.  All that she knows is 1) someone predicted that he'd be king and 2) the current king is going to be staying at her castle.  Therefore 3) she needs to become murderous.

I've never actually seen Macbeth on stage and now I'd really like to.  I watched a rather recent movie adaptation with Patrick Stewart in the title role.  I can't recommend it.  Lady Macbeth was played by Kate Fleetwood; I'm not familiar with her.  They played her almost like a horror movie villain and I thought that was too much.  She wasn't believable as a person.  Or at least not a person that anyone would choose to be in the same room with.
Having said that, I'm having trouble picturing just how that tightrope would be walked.  Her she is, a bloodthirsty Lady.  A contradiction of sorts.  Maybe she'd be secretly passionate but the really awful stuff would just bubble to the top when she couldn't hide it anymore?  Or maybe she really is just full evil at all times and poor Macbeth is somewhat bullied by her.  Or maybe there is some other reading that I'm not seeing.  (If I was a woman, I'd jump at the chance to play her once, just to see what I could come up with!)
I wonder if she is Shakespeare's most vicious main character?