Final answer:
Montresor's obsession with vengeance, premeditated planning, and lack of remorse in Edgar Allan Poe's story suggest that he is not mentally stable.
Step-by-step explanation:
We can infer that Montresor is not mentally stable from his disproportionate retribution against Fortunato in Edgar Allan Poe's The Cask of Amontillado. Signs of his instability are evident in the methodical and premeditated way he seeks to exact his revenge. The story reveals his obsession with vengeance and his cold, calculated approach. Montresor's own admission at the start of the tale, stating that he has borne many injuries until he vowed revenge, suggests a deeply troubled mindset.
Montresor's satisfaction with his own cunning and the lack of remorse even after many years indicate psychological disturbances. He shows no empathy or remorse for his actions and even takes pleasure in describing the details of his crime.