Poe uses dramatic irony—when the audience knows or understands something that some character(s) does not—in order to heighten tension as well as to help convey the theme that, often, people simply cannot accept responsibility for their wrongdoings. The narrator of the story, a murderer, says that his purpose is to write down, for all the world to see, "a series of mere household events." There is nothing "mere" and little "household" about the events that he relates! He tortures animals, eventually killing his once-beloved cat, and, then, in his attempt to kill the other cat, he murders his wife instead and then walls up her body in his basement.
To say that these are "mere household events" shows that the narrator either cannot or will not recognize his responsibility as well as how terribly egregious his actions are. We, of course, do (as does the author), and so this creates dramatic irony.
The story makes use of situational irony as well. If someone had just spent a long time...
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