In Macbeth, Lady Macbeth is instrumental in driving Macbeth towards his own destruction. She recognizes that he may not have the capacity to do what is required because he is "too full of the milk of human kindness" (I.v.14), meaning that he is essentially a good person. She intends to "unsex" (38) herself, removing all trace of any compassion so that she is tough enough for both of them. However, after having murdered Duncan, Macbeth finds an inner demonic strength which he knows stems from his "vaulting ambition" (I.vii.27) and the witches' words about Banquo's lineage which still haunt him. He cannot bear the thought that he has killed Duncan for Banquo's benefit. This compels him to continue his killing spree without any consultation with Lady Macbeth whom he believes will be impressed with his efforts.
In Act III, Macbeth has met with the murderers who will carry out Banquo's murder. In the meantime, Lady Macbeth who is now queen is anxious to talk to him and is waiting for him to come to her. It is during these moments of reflection that she ponders the lines from scene ii when she says:
Lady Macbeth: Nought'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. (III.ii)
Lady Macbeth and Macbeth have made much effort to get to this point and she is suggesting that "all's spent" because there has been great personal cost to them in attaining this position. There is nothing to be gained when that which is striven for does not bring the anticipated contentment. She knows that Macbeth is distracted and that there is much talk around the kingdom about who may have done what and she is therefore not enjoying her new-found status. She continues to say that sometimes it may preferable to be the one who has been destroyed or, in Duncan's case murdered because there will be no anxiety then and no feelings of guilt or "doubtful joy" such as she is feeling. She recognizes that the "destruction" which she has wrought has brought insecurities and made her question herself which is something that Lady Macbeth is not used to. She is used to getting what she wants and not concerning herself with anyone or anything else. Now she doubts herself and wishes that these feelings would end (such as they would have done were she in Duncan's position).
These lines belong to Lady Macbeth (3.2.6-9). Finally, she realizes how futile and hollow her (and her husband's) desire for power has been. "Naught had" equals "nothing had"; "all's spent" means that there is nothing left to bargain with; "Desire without content" means that even though she has gotten what she ostensibly wants (Duncan's death, the queenly crown), the price has been too exorbitant, the purchase dissatisfying.
"Tis safer to be that which we destroy," may mean that it is preferable to hold our baser instincts as possibilities of being rather than actual states of existence, an argument sustained by the end of this line, "than by destruction dwell in doubtful joy." Once realized, Lady Macbeth can no longer negotiate multiple interpretations of possible "selves." She sees what has become of letting her baser nature subsume her better instincts. There seems to be no returning to a purer self.