In "The Cask of Amontillado," how would you describe the persona that Poe has created for Montresor? Why might Poe have chosen someone like...

Montresor's character is defined by his extreme intelligence, cunning, and pride.  His personal pride and family pride combine to render him incapable of bearing Fortunato's "insult," whatever that may be.  He has already borne a "thousand injuries" at the hands of his nemesis, but however he has been insulted appears to have wounded his pride, and this means he must exact his revenge.  Further, his family pride compels him to live up to the Montresor...

Montresor's character is defined by his extreme intelligence, cunning, and pride.  His personal pride and family pride combine to render him incapable of bearing Fortunato's "insult," whatever that may be.  He has already borne a "thousand injuries" at the hands of his nemesis, but however he has been insulted appears to have wounded his pride, and this means he must exact his revenge.  Further, his family pride compels him to live up to the Montresor family motto: "You will not harm me with impunity" (translated from the Latin).  His family pride means that he must retaliate for all the injuries he has received; his personal pride means that he must retaliate in such a way that he can never be punished for it.  "I must not only punish," he says, "but punish with impunity."  His intelligence and cunning render him supremely capable of achieving this goal.  His vocabulary alone indicates how smart he is, and his ability to "smile in [Fortunato's] face" without Fortunato perceiving that Montresor "smile[d] now [...] at the thought of his immolation" conveys his capacity for dissembling.

Poe needs a character like Montresor because a character who was less intelligent or cunning would not be able to pull off this crime, and, likewise, he must be this proud or else he would never be incited or motivated to commit the crime.  He plans out the entire trap so perfectly, exploiting Fortunato's one "weak point," hiding his own face and person while in the street so that he could not be identified later, arranging it so that his servants will all be gone without actually telling them to leave, and surreptitiously urging Fortunato onward by insisting that he turn back so that he understands, in the end, that it was his own pride that Montresor effectively used as a weapon against him.