How do the last two paragraphs of Part III and the last paragraph of Part IV further lead the reader to question Montreso's sanity?
In the last two paragraphs of Part 3, Montresor drinks to "the long life" of Fortunato. By this time in the story, we suspect Montressor is going to kill Fortunato so the irony of this statement is obvious. In addition, these paragraphs call into question the sanity of Montresor because he is obviously toying with the ignorant Fortunato, much as a psychopath would do. In the last paragraph of Part IV, Montresor uses a play on the word "mason". Fortunato has alluded to the fact that he is a mason, a member of a respected secret society, and Montresor is not. At the beginning of the story, Montresor gives his motivation for killing Fortunato when he says Fortunato insulted him. The mention of the masons by Fortunato reinforces the motivation Montresor has for killing Fortunato because it reminds Montresor of the insults he has borne. However, as a cunning killer, Montresor continues to toy with Fortunato by producing a trowel, a tool used by common masons who build brick buildings. This pun continues the insanity of the game Montresor is playing with the unwitting Fortunato, who, in the end, is not so fortunate.