Your metaphor suggests you want quotes that show how evil and cunning Montresor is, qualities suggestive of a snake's perceived nature. Montresor shows both these qualities throughout the story. When he says, "I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation." Montresor goes to great lengths to give Fortunato no clue as to what he's planning, and he's able to hide his joy at the thought of Fortunato's death.
After Fortunato's drunkenness wears off, he tries to get out of his chains. This struggle pleases Montresor to the point that ". . . I ceased my labors and sat down upon the bones." He stops working so he can listen. This shows Montresor's madness in his desire to have Fortunato suffer before he dies.
One of the quotes that best illustrates Montresor's evil nature and his own madness is toward the end of the story. Fortunato begins to scream at the thought of what is happening to him, and Montresor gets such enjoyment from this that he "replied to the yells of him who clamored. I re-echoed--I aided--I surpassed them in volume and in strength."
" I must not only punish but punish with impunity." Montresor desires to punish Fortunato (for an unknown "insult") and get away with his crime.
"It must be understood, that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation." Montresor deceives Fortunato so that he has no clue of Montresor's evil intent.
"Pass your hand," I said, "over the wall ; you cannot help feeling the nitre. Indeed it is very damp. Once more let me implore you to return. No ? Then I must positively leave you." Of course, Fortunato cannot return; he has been chained to the wall of the cave. Montresor's remark is a cruel indication of Fortunato's doom.