What do you think was Montresor's motivation for killing Fortunato?

While the story contains no explicit mention of a motive, it may be gleaned somewhat from the narrator's choice of words and the dialogue. Montresor's motivation is complicated. It has to do not so much with murdering Fortunato specifically as somehow avenging the decline of his family name and prestige. Our narrator is a deeply insecure man. These insecurities manifest themselves as a desperate need for decisive, bold action, namely murdering Fortunato.

As to the...

While the story contains no explicit mention of a motive, it may be gleaned somewhat from the narrator's choice of words and the dialogue. Montresor's motivation is complicated. It has to do not so much with murdering Fortunato specifically as somehow avenging the decline of his family name and prestige. Our narrator is a deeply insecure man. These insecurities manifest themselves as a desperate need for decisive, bold action, namely murdering Fortunato.

As to the catalytic insult which triggered this whole thing? It most likely was something similar to the conversation the two characters had on their way through the wine vaults:

"The Montresors," I replied, "were a great and numerous family."

"I forget your arms."

Note that Montresor says they were "a great and numerous family." This implies the family has since dwindled and become less important. Montresor may logically feel ashamed or insecure about this, and thus he feels stung by Fortunato's lack of knowledge (or respect for) his family's importance. Fortunato does not know the family's arms or motto and does not seem to revere the Montresor name as Montresor thinks he should. Montresor resents Fortunato's arrogance and reads it as an insult to his entire existence—his name and family history.