Why was Montresor proud of his plan in The Cask of Amontillado?

Montresor is so proud of his plan because it was successful. He states in the last line of the story that for fifty years Fortunato has been walled up. Nobody has disturbed his final resting place.

Montresor must have conceived his revenge against Fortunato as a young man. He had endured, he says, a thousand insults and an injury. After the injury, he planned revenge. To him, revenge needed to be successful in two ways. First, the revenge had to go undetected by anyone other than the victim. Second, the victim had to know he was being punished and by whom. As Montresor puts it:

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.

Montresor succeeded in both goals. His crime went for fifty years without anyone being the wiser. Fortunato also knew in the end that it was Montresor walling him up to die. 

How does Montresor show pride in his crime in "The Cask of Amontillado"?

In “The Cask of Amontillado,” Montresor shows pride in his crime by pointing out his cleverness in plotting it and his painstaking care in accomplishing it. His excessive pride, or hubris, is also evident as he rationalizes his actions. To Montresor, his desire for revenge over Fortunato’s “injuries” and “insult” constitutes a valid motive for murder.

Part of the successful completion of the complex plan was taking his victim completely by surprise until it was too late and then realizing who controlled his horrible fate. The act of revenge, Montresor argues, would not actually have been accomplished if he had been found out. The idea of “impunity,” or being free of any consequences, is key. He states,

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 him wrong.

As Montresor describes how he built the wall and how Fortunato reacted, his pride in the stone construction and his gratification in knowing Fortunato’s terror are both revealed. When Fortunato begins to moan, Montresor listens to his cries with “satisfaction,” and he again mentions being “satisfied” when he realizes no sounds will penetrate the outer walls. Montresor details how carefully he builds and then fits in the last stone, with which he had struggled. Finally, his pointing out to the reader in the story’s very end that a half-century has passed is an indicator that he did act with impunity.