Montresor is mocking Fortunato. Montresor enjoys listening to Fortunato's pleas and cries. It shows the depth of Montresor's resentment (real or imagined) against Fortunato. He wants Fortunato to suffer, so he taunts him. It also shows the madness of Montresor, for would a sane person enjoy killing someone so much?
It shows Montresor's strange desperation in response to Fortunato, showing that he needed Fortunato more than ever but does not dare to reveal it to him but hide it in his inner courts of his heart more than he wants to reveal. He also claims to have a sick heart, so it means that Montreor's love for him was so deep that make him overcome with emotions and make him "sick" emotionally.
it means rest in peace