The line that you are referring to is, "You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat."
The story is told in the first person by Montresor. The "you" is not specifically mentioned during the story. Perhaps who "you" is, can be understood if you consider the last lines of the story, "In pace requiescat!"
The last lines mean rest in peace in Latin. At the end of the story, Montresor mentions that no one in half a century has disturbed where he placed Fortunato.
The "you" may be the reader, Montresor, or God. The story could be a form of confession- a way for Montresor to make amends with himself and God for what he did. I mention God because there is an exchange between Montresor and Fortunato toward the end of the story involving the phrase, for the love of God. After Fortunato exclaims these words, Montresor says, "yes" and also exclaims, for the love of God.
Asking why Montresor wrote the story may lead to who the "you" may be in the first paragraph.
"You" is addressed to readers who supposedly know the Montresor. He says, "You who know the nature of my soul...In other words, Montresor assumes that some people who knew or know him are going to be reading his account of the death of Fortunato. Of course, he expects others to read his story, also; otherwise he would not have continued with the idea that he did not threaten Fortunato but planned his death in secret and waited for the right moment to carry it out. The end of the story suggests that Montresor is obviously proud his deed and may be getting to the end of his life and he wants others to know what her did. He states that it has been 50 years since he killed Fortunato and he has not been caught . Thus, the last cut at Fortunato comes when he says, "Rest in Peace" in the last line of the story signaling his assurance of never getting caught.