"a certain shape of evil hue which by day may not be recognized, inasmuch as it may be a man of favored looks, or a fair and smiling woman; but by night becomes a misshapen beast with blood-bedabbled claws"
and I have to admit to wondering from the outset if perhaps we were going to be in for a bit o' the supernatural here, an idea that later seemed to be cemented by more than one mention of Poe, and of course, werewolves.
"... there are no secret entrances; the murderer was not hiding anywhere in the room; he did not go out by the window; he did not go out the salon door under my watching, nor the hall door under François' -- but he was not there when we entered. Yet a murderer had beheaded his victim there; we know in this case above all others that the dead man did not kill himself."
"a murderer who is utterly cold-blooded and cynical, and who firmly believes that these acts are done justifiably, to avenge wrongs. The crimes are the means of venting on the world a spite too deep for ordinary expression."