Here is my attempt at a paraphrase of Poe's typically nineteenth-century, florid prose into the less elaborated and more scaled-down language of our own age:
I had been able, as best I could, to put up with about a thousand different things Fortunato had done to hurt me. But when he openly insulted me, I knew I had to get back at him, to take revenge. You know me very well and know that I would never have allowed myself to threaten him directly. In time, I would get revenge—I knew that. But because I was so sure that I had to do this, I knew that the way I would do it couldn't have any risk associated with it. I had to punish him, but I had to do it in such a way that I wouldn't be found out and arrested. You don't achieve real revenge if you end up getting punished yourself as a result of it. You also don't achieve it if the person you're doing it to doesn't realize that you are the one who has carried out this retribution against him.
Though I may have taken a few liberties, I believe the above conveys in a more modern style the same thoughts that Poe expresses in that paragraph.