Think about whether or not Montresor is a reliable narrator in "The Cask of Amontillado."Do any details suggest that he might have imagined "The...

"The Cask of Amontillado" is Montressor's story. He is a
man who tells the story of his revenge on his apparent nemesis, Fortunato.  He does
claim that Fortunato has committed a thousand injuries until he (Montressor) has had
enough.  We either have to believe him and take him literally at his word, or we have to
test his reliability as a narrator and see those thousand injuries as hyperbole
(exaggeration).  I'll point out a few things, you watch for exaggerations, and you can
make your own judgment from there.

Montressor claims a
thousand injuries, yet when he is offered just one "insult," he is ready to seek
revenge--to the death. Any exaggeration there?

When
Montressor meets Fortunato for the first time at Carnival, he says
this:

I was so
pleased to see him, that I thought I should never have done wringing his
hand.

This is a detail that
doesn't really matter, yet Montressor feels the need to exaggerate even about a
handshake.

Once Montressor has gotten the coughing
Fortunato into the underground vaults, he says the
following:

readability="13">

"Come," I said, with decision, we will go back;
your health is precious. You are rich, respected, admired, beloved; you are happy as
once I was. You are a man to be missed. For me it is no matter. We will go back; you
will be ill and I cannot be
responsible.

This is an
obvious exaggeration used to make continue luring his enemy into the catacombs of his
villa. 

If you've read the entire work, you know that
Montressor is not above saying anything he needs to in order to get what he wants and
convince you he's not really insane, just protecting his family honor (another
exaggeration, of course). 

Perhaps the most compelling
argument that whatever Fortunato did was minimal or even inadvertent (accidental) and
completely one-sided is the fact that Fortunato doesn't even really hesitate to go with
Montressor.  Surely an enemy worthy of murdering will have shown some real animosity,
but that doesn't happen in this story. 

Your assessment is
valid.  Follow this train of thought and you can make a strong and effective
argument.