One point of view is that the narrator is telling the story of Usher, not his own story. As such, Roderick Usher is the protagonist of the tale. In Poe literature, the protagonist need not be an admirable hero. And indeed, if Roderick knew his sister was alive when he put her in the tomb, then he is anything but admirable.
Another point of view is that the narrator is the protagonist of “Usher.” His “journey” is from naiveté to understanding of the potential of the disturbed mind of Roderick Usher. As readers we are invested in his tale, his transformation, and his escape from the House of Usher more than we are invested in what actually happens to Usher (which, we know from beginning, must be death).