What is meant by, “the private ‘I’ that each of us adds to, and discovers in, the shadows cast by [Poe’s] tales and poems”?

Poe's protagonists are often crazy. The reader, however, can find a shred of this craziness that the reader can relate to. The reader may secretly probe their imagination and wonder how they might commit a senseless crime, or how they might dispose of the body. Of course these thoughts are not to be taken seriously but Poe's stories let us go to these dark places in our imagination.