So, there's a really great million-dollar word that applies to Mrs. Sparsit: schadenfreude (pronounced sha-den-froi-da). It's an awesome term we borrowed from German, and it means "taking pleasure in the misery and misfortune of others." Use it the next chance you get (correctly! In the right context!) to impress your friends and wow your teacher. Anyhow, Mrs. Sparsit is completely immersed in trying to get Louisa to cheat on Bounderby with Harthouse. In her mind, she pictures the process of seduction and, hopefully, illicit sex, as Louisa walking down a long staircase into a giant pit of doom and sin. The staircase imagery is pretty powerful. It contrasts the civilized world (someone had to build that staircase, and architecture tends to be manmade) and the bestial nature of sex and desire (the hole of despair at the bottom is shapeless and not of this world).
We could probably take it a step farther, and think about the way this staircase to Hell compares with the Biblical image of Jacob's ladder to Heaven (Jacob sees it in a vision in the Book of Genesis), and the similar ideas about it taking small, incrementally staggered steps to get to these places.