The reader/auditor does not know what Fortunato has done to anger Montresor. So, there's no real way to know if Montresor's anger is justified. In any case, Montresor is certainly vengeful and his attitude is dictated by this need for revenge. Montresor is completely cold in killing Fortunato and is likewise cold and stoic in the retelling of the tale. His mindset is bent on revenge at all costs but his outward demeanor in dealing with Fortunato and in telling the tale to the reader is stoic (without emotion). So, Montresor is cold and calculating. Montresor even takes quiet pleasure in his enemy's demise. As he is walling Fortunato in, he considers listening to the rattling of the chains: 

I laid the second tier, and the third, and the fourth; and then I heard the furious vibrations of the chain. The noise lasted for several minutes, during which, that I might hearken to it with the more satisfaction, I ceased my labors and sat down upon the bones. 

Montresor goes about his thirst for revenge quietly and workman-like. This makes for an odd combination of extreme revenge carried out with an emotionless determination.