Quote 19
First of all I dismembered the corpse. I cut off the head and the arms and the legs. (8)
We don't bring this up to be gross, but to point out that this violent image is perhaps more disturbing than the image of the man smashed by the bed. For one thing, it's much more concrete and easy to visualize. It's also more invasively violent, even though the man is (we hope) already dead for this process.
Quote 20
It took me an hour to place my whole head within the opening so far that I could see him as he lay upon his bed. (3)
This strange time management technique suggests extreme loneliness. We learn later that the narrator has problems sleeping at night (and probably during the day). This moment is when we first begin to notice there's something funny going on with time.
Quote 21
And this I did for seven long nights – every night just at midnight. (3)
It seems Poe is reminding us that we are in fact in a Gothic tale, where all bad things have to wait until just after midnight before they can play.