The Puzzle of Dicken's Last Plot

What can all this mean? We have been told that, shortly before Christmas Eve, Jasper took to wearing a thick black-silk handkerchief for his throat. He hung it over his arm, "his face knitted and stern," as he entered his house for his Christmas Eve dinner. If he strangled Edwin with the scarf, as we are to suppose, he did not lead him, drugged, to the tower top, and pitch him off. Is part of Jasper's vision reminiscent--the brief, unresisting death--while another part is a separate vision, is PROSPECTIVE, "premonitory"? Does he see himself pitching Neville Landless off the tower top, or see him fallen from the Cathedral roof?