What evidence is there that Lady Macbeth is not as strong as she would like to be?
An early indication that Lady Macbeth is not as strong as she would like to be can be found in these lines in Act 2, Scene 2, spoken while she waits for her husband's return from Duncan's chamber.
Alack, I am afraid they have awaked,
And 'tis not done. Th' attempt and not the deed
Confounds us. Hark!--I laid their daggers ready;
He could not miss 'em. Had he not resembled
My father as he slept, I had done 't.
The word "Alack" expresses fear and something like despair. She is a nervous wreck. She is afraid the grooms may have awakened and perhaps seen her husband hovering over the King's bed. She is apparently rationalizing when she tells herself that she would have killed Duncan herself. She has to think of a reason why she couldn't bring herself to do it. She was in the chamber, saw the grooms and the King sound asleep, and the idea of committing the murder herself must have crossed her mind. But her husband is much better at killing people than she is. He has had plenty of experience. Lady Macbeth is not as strong as she would like to be.
The best evidence for this is that Lady Macbeth eventually cracks under the enormous weight of her conscience, a development that results in her eventual suicide. At the beginning of the play, she asks "the spirits that attend mortal thoughts" to "unsex" her, a request that, during Shakespeare's time, would have connoted the loss of mercy and other virtues associated with women. Indeed, she is utterly ruthless in pushing Macbeth into the murder of Duncan, and shows no remorse after the fact. But by the beginning of Act V, with the Macduff family and Banquo added to the list of murders, her conscience seems to have overtaken her will. In the famous sleepwalking scene, she ceaselessly tries to scrub her hands clean of imagined blood, shouting "Out, damned spot!" This is clearly her conscience haunting her, and it has left her a broken woman, a shadow of the strong, chillingly ruthless conspirator at the play's beginning. She kills herself just before the final battle of the play.