Why does Montresor choose to retell his story some half a century after the actual event?

It is not exactly clear under what precise conditions Montresor is recounting the events of that terrible day. It may be some kind of confession, or it may not even be a true story. Who knows? The important thing is that Montresor did not simply want to wreak revenge on Fortunato; he wanted to get away with it. And with the passage of fifty years since he committed his act of murder, we can be sure that he now feels confident enough to admit what he has done. There is no doubt that, after all these years, Montresor still feels immense pride at getting revenge on Fortunato. But at the same time, he wants to make absolutely certain that he is not going to have to pay for his crimes.