Consider! According to the story given to us, the assassin had less than a minute after the murder had been committed to take that ring, which was under another ring, from the dead man's finger, to replace the other ring--a thing which he would surely never have done--and to put that singular card beside his victim.

I say that this was obviously impossible.
"You may argue--but I have too much respect for your judgment, Watson, to think that you will do so--that the ring may have been taken before the man was killed.

The fact that the candle had been lit only a short time shows that there had been no lengthy interview.

Was Douglas, from what we hear of his fearless character, a man who would be likely to give up his wedding ring at such short notice, or could we conceive of his giving it up at all?
No, no, Watson, the assassin was alone with the dead man for some time with the lamp lit.

Of that I have no doubt at all.
"But the gunshot was apparently the cause of death.