First, Montresor knows an ultimate weakness of Fortunato:
He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine.
Montresor is thereby able to exploit this weakness in his plans to kill Fortunato while also maintaining a keen ability in holding the man's trust:
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.
While Montresor plots to kill Fortunato by using personal knowledge against the man, he is simultaneously able to never give himself away; Fortunato trusts him, and even as he is led to what will become his tomb, he rejects Montresor's insincere attempts to turn back. Thus, he allows himself to be led directly to his death.
In Shakespeare's Julius Caesar, there is a similar betrayal. Brutus knows that Caesar trusts him, yet he aligns himself with the forces who seek to kill Caesar. It is Brutus's betrayal that pains Caesar most and the one which is the literal final blow:
Et tu, Bruté? —Then fall, Caesar.
In this case, the two are closer friends than Montresor and Fortunato, yet Brutus clearly violates a similar sense of trust in order to commit murder.