(I have edited your original query since only one question is allowed per eNotes post.)
It is clear that Montresor's hatred of Fortunato is so overwhelming that he is willing to commit the ultimate act of revenge--murder--in order to settle the score. The reader never discovers the true source of Montresor's hatred, only that
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
The two men must have been long acquaintances, and Fortunato may have even considered Montresor a friend, though he never calls him such during the story. Fortunato's stature and wealth seem to have led him to consider Montresor a lesser man: Fortunato mocks him when he claims to be a fellow Mason, and he makes it clear that Montresor's family is not equal to his own. Fortunato obviously trusts Montresor--though his drunkenness has undoubtedly contributed to his misplaced trust--and he willingly follows Montresor into the eerie catacombs on the promise of a rare bottle of Amontillado. Montresor repeatedly refers to Fortunato as "my friend," but it is only a ruse. He knows that Fortunato's inebriation and love of the grape is such a weakness that Fortunato will follow him anywhere. Montresor knows Fortunato will not be missed immediately during the "supreme madness of the carnival season," and that his victim's wife has decided to stay at home. Even in Fortunato's final moments of life, he cannot perceive that Montresor means to complete his act of murder; instead, he believes it is but an "excellent jest," and that they will share "a rich laugh" over it later in the evening.