Lady Macbeth’s character has an ending that was unexpected. Why is her character’s demise so unexpected?

Lady Macbeth eventually goes mad and commits suicide. We see the extent of her madness in act 5, scene 1. Lady Macbeth is sleepwalking, talking in her sleep, and trying to wash her hands of bloodstains that she cannot remove. Her actions in this scene represent her guilt for her role in planning and helping Macbeth execute Duncan's murder. The reason this is so unexpected is that Lady Macbeth's character seems much stronger earlier in the play.
In the first few acts, Lady Macbeth is very assertive and in control. She is her husband's true partner in the sense that he confides in her when he learns he will be king some day, and they work together on the plan to kill Duncan. However, Lady Macbeth is the more ruthless character. She worries that Macbeth is too "full o' th' milk of human kindness" and calls upon "spirits" to "unsex" her, or rid her of her feminine qualities so she can be as violent as men can be (I.v). After Macbeth kills Duncan and is obviously disturbed by his actions, he cannot bear to go back and plant the knives on the guards; it is Lady Macbeth who goes back to the scene and finishes the job. Lady Macbeth is the stable partner in the marriage on the night of Macbeth's coronation banquet when he sees Banquo's ghost and seems to be going mad before everyone's eyes.
Because she seems so in control throughout the early acts of the play, and because we don't hear from her much once Macbeth starts planning to have other characters like Banquo and those of Macduff's family killed, it comes as a shock to the audience that Lady Macbeth is so far gone in act 5. She seems even more affected by guilt after their crimes, while Macbeth is desperate and will go to any extremes to keep his power.