What does Montresor admit is his motive for his crime in "The Cask of Amontillado"?

Montresor has "vowed revenge" for what he calls the "thousand injuries" of Fortunato.

What these injuries are the reader never learns. Instead, Poe's unreliable narrator describes how revenge must be planned so that there can be no element of risk. According to Montresor, revenge is only complete when it is done "with impunity"; that is, there are no consequences felt by the avenger for his act. Yet, while the avenger must remain unknown to the authorities or any one besides the victim, the victim must be made aware of the avenger in order for the act to be truly revenge.

Poe's narrator enacts his revenge precisely according to his blueprint, luring the vain and arrogant Fortunato into the Montresor vaults where, supposedly, a large cask of Amontillado, a variety of sherry, is stored. Because it is the Carnival season, it is unlikely that any of the revelers will pay attention to Fortunato's departure; moreover, people are all in costume, so recognition of perpetrator and victim is difficult, if not impossible. In addition, Montresor's servants are gone, so they cannot know what their master has planned.

As they enter the vaults, Montresor picks up two lighted torches and gives one to Fortunato, pointing out to him the dampness of the walls and the niter upon them. When Fortunato coughs in this damp atmosphere, Montresor feigns concern, revealing a vague sense of his resentment while playing to his foe's ego: 

"Come...we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi--"

With dramatic irony Fortunato replies, 

"Enough! The cough is a mere nothing; it will not kill me. I shall not die of a cough."

Finally, after a series of twists and turns through the niter-covered apertures, Montresor lures Fortunato into a small recess by furthering his intoxication with Medoc. Then, Montresor fetters his victim to the granite in which two iron rings hang. With stone and mortar, he walls up the entrance to this niche.
His revenge completed, Montresor boasts to his audience that for fifty years no one has discovered his crime. Therefore, he has achieved perfect revenge.