Lady Macbeth is a complex character. The main constant in her character is her strong sense of duty. She sees as her first duty as a wife supporting her husband and being responsible for his success, being a sort of power behind the throne. On the other hand, an important part of what she understands as her role as a woman is to be tender, empathetic, and a moral compass. In order to support the ambitions of a husband too filled with "the milk of human kindness," she must temporarily suppress or repudiate her feminine nature. In a soliloquy she states:
... Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me ... full [o]f direst cruelty.
The contradiction between the strength Lady Macbeth needs to commit evil acts and her feminine nature drives her insane as the play progresses, and her strength ultimately gives way to remorse. One could say that in the beginning of the play she succeeds by strength of will in "unsexing" herself but that her feminine nature (as femininity was conceived by Shakespeare) eventually reasserts itself as the play progresses.