How does lady Macbeth act fair when she meets king Duncan? Why is she really foul inside

Lady Macbeth pretends to be the the gracious host to Duncan,

Your servants ever
Have theirs, themselves, and what is theirs in compt,
To make their audit at your highness' pleasure,
Still to return your own.

Of course she plans to be "the serpent underneath" by killing Duncan during his stay.