Although Lady Macbeth and Macbeth conspired together, at what point and why did she exhibit abnormal behavior?
Certainly by the beginning of Act 5, we know that a doctor has been called to see what is ailing Lady Macbeth. The war that will eventually unseat and kill Macbeth is at hand, and his wife's mind is becoming unhinged:
Since his majesty went into the field, I have seen her rise from her bed, throw her night-gown upon her, unlock her closet, take forth paper, fold it, write upon't, read it, afterwards seal it, and again return to bed; yet all this while in a most fast sleep.
This and worse, of course. She is consumed by guilt; she tries to rub her hands free of the blood that can never be washed away.
But even by Act 3, scene 2, we see that she knows that all she has hoped for has elluded her and will never be hers. The murder that she planned is beginning the weigh upon her:
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.
By the end of the play, it is learned that she has taken her own life.