How does poe create an unsettling atmosphere with stylistic devices in the passage above
down—steadily down it crept. i took a frenzied pleasure in contrasting its downward with its lateral velocity. to the right—to the left—far and wide—with the shriek of a spirit! to my heart with the stealthy pace of the tiger! i alternately laughed and howled, as the one or the other idea grew predominant.

I would say the part about "down-steadily down it crept" and "frenzied pleasure",  at its' downward descent is unsettling because of the negative symbolism of something going down instead of up. Also 
"To the right—to the left—far and wide—with the shriek of a spirit! to my heart with the stealthy pace of  the tiger! " This passage also indicates  a troubled spirit swinging back and forth and shrieking and creeping like a tiger as a tiger is a symbol of aggression and conjures up an image of being stalked by a tiger.