Why does Montresor adopt a feigned innocence in his account? For instance, he describes encountering his friend as if it were a chance...

Montresor narrates the story with the tone of an innocent man, because he feels no guilt for what he did 50 years ago.  He lures Fortunato into the wine cellar because of some unknown slight against him.  Montresor never states exactly what the insult is, but does state that he must,

 ‘‘punish with impunity. ‘‘Although Montresor's explains that Fortunato has committed a "thousand injuries’’ and a final "insult,"

Montresor has not lost one moment of sleep over this horrible act, because he has justified his behavior through the excuse of avenging his name.  Fortunato had somehow insulted, or committed the "thousand injuries" and Montresor decides to get even.  When he is narrating the story for the reader he tells the events with no guilt and with no pride.  He simply relates the facts.  He is not asking for forgiveness and he is not bragging.  This method, by Poe, makes Montresor seem even more dangerous than he probably is, but as the reader, I didn't want to meet him or cross a character like Montresor.