A+Thousand+Injuries...

This quote serves multiple functions. On the first read, it tells us that Fortunato wronged Montressor and Montressor will get him back. It is equally important to note that Montressor must get his revenge in such a way as to neither be punished for it or receive no public attention to his besting Fortunato. In other words, the world must know that Montressor one, and he can't catch any flak from it.

As you reread the quote, you might notice that Montressor speaks to the reader directly; a common happening in regards to first person narrations. This can be interrprited to mean that the speaker is self aware enough to know that his writing will be read, and so he establishes a rapport with him or her. Discounting this theory is that we supposedly "know the nature of his soul". Two ways that this can lead are as follows: either he is writing this to someone that knows him in his setting, or he is writing with the idea that we know his soul, because we share his soul. Montressor and the reader are alike, and suffer the same pangs of humanity and the desire for revenge.