The hyperbole is the description of a “thousand injuries” and it does not make the reader feel sympathy for Montresor.
Since hyperbole is the use of exaggeration to make a point, we are looking for an instance when the narrator, Montresor, exaggerates and the author is using this exaggeration to point out something about his character. In fact, Poe begins the story with hyperbole because he wants this exaggeration to really grab the reader.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat.
The first sentence is the sentence that really contains the hyperbole, but I include the second one because it points to Montresor’s state of mind. This is hyperbole because it describes a “thousand injuries” and that is a little extreme. It is so extreme that Montresor vows “revenge,” which is also kind of hyperbole, or normally is when you are not actually murdering someone for it.
Normally, this would get the reader sympathetic. Oh, you have been done a thousand injuries? That’s terrible? I feel so bad for you. However, the fact that Montresor is acting mad might give the reader second thoughts about feeling any sympathy. Montresor doesn’t name any specific injury, for one thing. He also says that he is going to get revenge, which does not exactly engender sympathy. In case you had any doubt, all you had to do was keep reading.
I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.
This implies that he not only needs to punish Fortunato for whatever it is he did, but he has to get away with it for it to be worth it. Again, this does not make the reader feel sympathy for him, and adds to the feeling that he is crazy. He is a murderer after all. Readers are not going to have sympathy for a murderer except in extreme situations where the narrator is very specific. If Montresor had said that Fortunato killed his entire family or something, then we would have had sympathy.
If anything, the reader feels sympathy for Montresor because he is a madman, but because he needs to get revenge on Fortunato for some unknown and likely imagined injury, we do not. The story is a good example of why you really need to be careful who you hang out with, and definitely avoid crypts on carnival, or Halloween.
Poe is the master of unreliable narrators. In this story, a madman tells us how he planned the perfect murder, and got away with it. Oh, he tricked Fortunato into the crypt, buried him alive by bricking him in, and no one was ever the wiser. This was all because of some imagined insults that were blown up in his own psychotic mind. To this narrator, everything is hyperbole.