What irony, characterzation, and imagery in the story supports the insanity theory that Montresor is psychologically unstable?"The Cask of...

Characterization

Immediately, Montresor's
obsession is presented as  "the thousand insults" of Fortunato that he will avenge in
the only proper manner: 

• punishment must come
  with impunity


• the retribution must not take over the
  redresser


• the avenger must make himself known to his
  victim

His explanation of how he has dissembled
seems rather unbalanced.  For, Montresor states that when Fortunato accosts him in his
motley costume,

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

Montresor banters with
Fortunato, calling his attention to the "white web-work" of the walls while at the same
time encouraging Fortunato, leading him farther into the catacombs, then objecting to
the unlucky man's progression in an affected concern for his health, while at the same
time ridiculing his cough, "Ugh!ugh!ugh!ugh!....." and calling Fortunato "friend." After
this, Montesor makes a pun upon the word mason, ridiculing Fortunato's pride at being in
the society of Freemasons; subsequently, he feigns concern for Fortunato as he says,
"...let me implore you to return."
[irony,also] 

At this point, there are possible sexual
innuendoes, suggestive of the perverseness of Montresor's plan for revenge.  Before
Montresor begins building his wall, the tettered Fortunato "ejaculated" the words " The
Amontillado!" then Montresor hears a low, moaning sound from Fortunato and ceases his
work, sitting down.  When Fortunato begins to scream, Montresor narrates that he began
to "Unsheath my rapier," and "grope with it about the recess."  Certainly, the use of
sexually suggestive language points to the perversity and sadism of Montresor who also
mocks Fortunato in his desperate cry, "For the love of God!" as he
himself cries the same words.

After all this, the horror
that Montesor feels is the horror within him; the evil of which he has been capable, and
he takes fifty years to tell his
tale.

Imagery

There
is much sight and sound imagery suggestive of imbalance. Regarding
sight--

• the harlequin costume of
  Fortunato


• the insufferable dampness of the
  vaults.


• the bones scattered
  "promiscuously"


• the coat of arms with a golden foot
  crushes a serpent who fangs are imbedded into the knight's
  heel


• the chains and
  padlock


• the rapier


• the
  stones of the trap


• the flambeaux that cast only a few
  rays upon Fortunato


• the torch that Montresor thrusts into
  the remaining opening to the tomb that he finally
  closes.


• the rampart of
  bones

regarding
sound--

• the jingling of the fool's cap on
  Fortunato acting as an ironic foreshadowing of what is to
  come.


• the drops of moisture trickling among the
  bones


• the low, moaning cry of
  Fortunato


• the silence after Montesor finally calls out
  "Fortunato!"

Irony

Montresor's
carefully executed betrayal and retribution against Fortunato are meshed and intensified
in meaning with the skillful and pervasive use of
irony.

• The first irony occurs with Montesor's
  intentional telling the servants that he will be gone so that they will run off.  This
  reverse psychology illustrates the skewering of
  relationships.


• He toasts to Fortunato's long
  life


• He expresses concern for Fortunato's
  health


• He waves the trowel, saying he is a mason, giving
  an ironic twist to Fortunato's use of the word.Shortly afterwards, he points to his coat
  of arms that states that "No one provokes me with
  impunity."


• He assures Fortunato that he will not die of
  his cough


• He gives "for the love of God" another
  meaning.