A good thesis statement on this subject would be direct, focused, succinct, and open to argumentation. For instance, you could say something like:
The character of Montresor in "The Cask of Amontillado" is an obvious example of a person with psychopathic tendencies.
Psychopaths are people with mental disorders that cause them to act completely without consciences. They feel no remorse, guilt, or empathy with others. They can be very deceptive, however, and seem to be charming, friendly, caring, and trustworthy. They have a tendency to carefully plan out their actions and manipulate people. They are not always violent, but they have no qualms about using violence to achieve their goals.
Montresor, of course, fits this definition well, and there are numerous examples in the story to support this. For instance, he manipulates Fortunato into accompanying him into the catacombs with the story about the Amontillado, along with flattery about Fortunato being a connoisseur of fine wine. He continues to be charming and friendly even as he lures Fortunato to his doom. He has coldly prearranged that there is no one around who will see them descend into the vaults together. He drinks to Fortunato's long life even as he is about to bury him alive. After he has shackled Fortunato to the wall and is bricking up the entrance, instead of being touched by Fortunato's pleas, Montresor continues to mock him. Finally, when the deed is done, Montresor says that his heart feels sick not because of the murder he has just committed, but merely from the dampness in the vaults.