Do you think the insult really happend or did it just exist in Montresor's head?
This is a subjective question. Many readers will see the answer to this question differently based upon personal interpretation. Therefore, the answer to the question will be subjective as well.
Montresor, from Poe's "The Cask of Amontillado", has spent the past fifty years contemplating the insult enacted upon him by Fortunato. One could look at this in two very different ways.
First, one could justify that Montresor is insane. His insanity stems from the fact that he has been obsessed with the insult placed upon his name by Fortunato. From this obsession stems insanity.
Second, one could look at Montersor's desire to seek revenge on Fortunato as a simply act. Fortunato wronged Montersor and, therefore, deserved death.
Lastly, one could examine Montresor's reaction to impending concealment in the tombs. Fortunato begins to scream. Montresor suddenly worries that someone may hear the screams. Montresor stops bricking up the wall. He becomes fearful of being caught. One could look at this in two different ways. First, Montersor is conscious of what he is doing is wrong; therefore, he is sane. Or, secondly, Montersor is insane given he begins to scream along with Fortunato.