Lady Macbeth speaks this line in Act III, Scene 2, line 5. Prior to this, she asks a servant to tell Macbeth she needs to speak with him. The lines reflect her thoughts before Macbeth enters. Lines 4 -7 are:
Naught's had, all's spent,
Where our desire is got without content.
'Tis safer to be that which we destroy
Than by destruction dwell in doubtful joy.
Lady Macbeth feels that nothing has really been gained by killing Duncan because even though she and Macbeth got what they wanted, it wasn't worth it because they can't be truly happy about it. She feel it would be better to be dead like Duncan than to have to live in the uncertainty of what will happen because of their crime. She says killing Duncan was really a wasted crime because of the uncertainty.