Lady Macbeth's speeches are concise, sharp, ambitious and finely specific and pointed. THey characterise her as concise, sharp, ambitious and dangerous. Having read the letter, she gets straight to the point: and she often speaks in these punchy lists:
Glamis thou art, and Cawdor, and shalt be
What thou art promised.
This is a highly methodical mind, constantly speaking in antitheses used to set out her information:
What thou wouldst highly,
That wouldst thou holily; wouldst not play false,
And yet wouldst wrongly win.
Highly/holily... false/win. This is an organised, clear-thinking mind. And it is one capable of coming to a decision fast.
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements.
Even before Duncan is there, she's decided he will be dead.
"Fair and foul" is precisely the nature of the play. Things are both at once. Macbeth is a loyal subject, a deserving warrior, but also a secretive, plotting murderer. Macbeth is a thuggish "dead butcher", but also someone capable of beautiful, poetic thoughts. Lady Macbeth is a great wife, an ambitious partner, but also a homicidal murderer. Lady Macbeth is a murderer, but also scared to murder because Duncan resembled her "father as he slept". Things are both good and bad.
The day of battle, as Macbeth says, has "foul" weather, but is a "fair" day because they have won the battle. And the witches' prophecies obviously follow the same pattern: Banquo will be "lesser than Macbeth and greater", "not so happy but much happier" ... and so on.