Final answer:
In Shakespeare's 'Macbeth', Lady Macbeth dies due to a mental breakdown and a guilty conscience, as she is overwhelmed by guilt for her role in the murder of Duncan.
Step-by-step explanation:
The character who dies due to a mental breakdown and a guilty conscience in Shakespeare's play 'Macbeth' is Lady Macbeth. Lady Macbeth conspires with her husband to murder King Duncan and eventually becomes consumed by guilt over their actions. Her guilty conscience leads to sleepwalking episodes and hallucinations, wherein she tries to wash the imaginary bloodstains from her hands, reflecting her deep psychological torment. Ultimately, her overwhelming guilt and deteriorating mental state lead to her demise.