In Act III, Scene 2, lines 6-9, Lady Macbeth's lines denote that she feels that nothing is gained if a person attains what he wants, but is still not content. She feels it is probably better to be the person who is murdered than the killer who is tortured by anxiety and paranoia.
In a sense, Lady Macbeth seems to acknowledge, as Shakespearean critic Harold Bloom notes, the idea that people "are lived, thought, and willed by forces not [them]selves." Certainly, Macbeth seems driven by the forces of violence and paranoia. When he enters, Lady Macbeth asks her husband why he remains alone so much. She encourages him to stop worrying about what is done--"what's done is done"(3.2.10)--but he tells her, "We have scotched the snake, not killed it" (3.2.15). That is, Macbeth fears that someone might try to murder him as he has murdered King Duncan.
Lady Macbeth's quote speaks to how she feels about her husband's situation: nothing is gained by Macbeth if he is not satisfied. Instead of being happy with his success, Macbeth is tortured by anxiety that he has other enemies whom he has not eliminated. Lady Macbeth seems to realize how hollow their victory in having acquired the crown of Scotland has been.
Additional source: Bloom, Harold. Shakespeare: The Invention of the Human. Riverhead Books. 1998.
Lady Macbeth, in Shakespeare's Macbeth, echoes an idea also stated by her husband. They have power, but they do not safely have power.
Lady Macbeth expresses discontent with her situation. She has fulfilled her desire (Macbeth is king), but the fulfillment has brought no contentment. She's figuratively spent herself, but achieved nothing for her efforts. Any joy she feels is contaminated.
Lady Macbeth would rather be that which they destroyed (Duncan) than to live with the uneasy power they've achieved. At least Duncan has peace.
In short, Macbeth has shut her out of the decision-making process and shunned her since the assassination. He continues to foul up their cover up by doing things like ordering the killings of Banquo and Fleance. Macbeth has become a tyrant and people are beginning to suspect him/them of treachery. She has power (or at least Macbeth does, since he's replaced her as adviser with the witches), but that power is not solidified. She has no peace, which the dead Duncan does.