Why was Montresor so intent on seeking revenge against Fortunato in "The Cask of Amontillado"?

Montresor wants revenge against
Fortunato for some imaginary insult.

It is important to
remember that Montresor does not have a real reason for killing Fortunato.  Montresor is a
madman, and the reasons he wants to kill Fortunato are all imaginary.  Poe makes sure that we
realize this with his very first sentence.

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 gave
utterance to a threat. At length I would be avenged €¦

Theis our first clue.  A thousand injuries?  Really?  Isnt that a little extreme?  You
cant really do a thousand terrible things to a person, especially without him noticing. 
Montresor probably imagines these, or turns minor slights and insults into crimes worthy of
prosecution.

Montresor wants to not get caught.  He explains that he has to
get away with the murder, or he will not really be avenged.  Since the telling of the story is
fifty years after the event, he must have succeeded.

Another reason that we
know that Fortunato did not really do something terrible enough to Montresor to justify being
killed is that he agrees to go with Montresor into the catacombs.  If you had wronged a person
in some terrible way, you would not go underground with him at night without
witnesses.

Montresor is able to easily convince Fortunato to go into the
catacombs with him by telling him he has a cask of valuable Amontillado wine that he needs his
opinion of.  When he offers to show it to Luchesi instead, Fortunato protests.

Come, let us go."

"Whither?"

"To your vaults."

"My friend, no; I will not impose upon your good nature. I perceive you have an
engagement. Luchresi--"

"I have no engagement;
--come."

Montresor makes other arguments that he
should not go, such as the fact that it will not be good for his cold, but Fortunato insists. 
He willingly goes underground with Montresor because he does not know that the man is his
enemy.

If Fortunato had really done something to Montresor, it would be a
very different story.  This one is the tale of a crazy guy killing another guy for no reason.
 He is a psychopath, because he believes that he is right.  Montresor really thinks that
Fortunato deserves to die and has no problem with killing him as long as he gets away with
it.