Montresor tells the reader that Fortunato has done him a thousand injuries.
We are never actually told what Fortunato did.
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
It seems that Fortunato has no idea that Montresor is angry with him, or he would not have agreed to go underground. Therefore the “injuries” are likely small and imagined, such as a supposed snub or unintentional rudeness interpreted from an innocent action. Montresor makes it clear that he never uttered a threat or gave Fortunato cause for concern. Instead, he pretended to be his friend and slowly planned the perfect murder.
Fortunato seems to be more highly placed in society than Montresor. After all, he is a Mason and Montresor clearly is not (although he pretends to be). Therefore the real reason Montresor kills him is probably because he is jealous and feels that he deserves what Fortunato has.
Montresor is a madman. He could be driven by the slightest of actions, which is his mind becomes a huge injury that is not to be borne. Fortunato did not stand a chance, because once Montresor's delusions set in he was a goner.