In this scene, Lady Macbeth receives a letter from Macbeth in which he tells her about the prophecies and his recent promotion to Thane of Cawdor. Shakespeare's portrayal of Lady Macbeth in this scene is uniformly negative. Specifically, he presents her as an ambitious and ruthless woman, capable of convincing Macbeth that killing Duncan is the only way to realize his dreams of becoming king.
To demonstrate this, take a look at her dialogue after receiving the letter. She talks about Macbeth lacking the necessary feelings ("the illness") to go for the crown. This is because Macbeth is too kind-hearted ("too full o' th' milk of human kindness.") In contrast, Lady Macbeth urges Macbeth to return home so that she can use her influence to ready him for murder:
Hie thee hither,
That I may pour my spirits in thine ear
And chastise with the valor of my tongue
All that impedes thee from the golden round.
This implies that while Macbeth lacks the cunning and cold-bloodedness to kill, Lady Macbeth has it in abundance and, more importantly, can allay any of his fears about committing such a heinous crime.
Moreover, by using the words "golden round" to describe the crown, Shakespeare shows that Lady Macbeth is just as eager for power as her husband.
Finally, after the servant brings news of Macbeth and Duncan's arrival, Lady Macbeth asks to be unsexed and to be filled with "direst cruelty." In other words, she wants to possess all the traits which make murder possible so that she can guarantee their accession to power and prestige.