The audience is largely unaware of what motivates Lady Macbeth to persuade her husband to murder King Duncan. In Act 1 when Lady Macbeth receives the letter from Macbeth informing her of the witches' prophecy and his recent promotion to becoming the Thane of Cawdor, she is excited that all these good things are happening for Macbeth, and indirectly, for her. But she immediately considers that Macbeth is too kind to do any ill deeds to help himself further his position. It seems that Lady Macbeth wants security and a guarantee that Macbeth will in fact become king and she queen. It is unclear why she just does not wait for fate to take its course, especially since the first part of the witches' prophecy came true without Macbeth's having to do any ill deeds. So it appears that some inner greed and/or desire for security motivates Lady Macbeth.