How and where does the battle between good and evil take place in Stevenson's The Strange Case of Dr Jekyll and Mr. Hyde?

In Chapter 8, entitled
"The Last Night," Poole, Jekyll's butler, calls Utterson to Jekyll's house because he
is so concerned about his employer.  Poole fears that someone has murdered Jekyll, and that it
happened some eight days ago, he says, "'when [the household] heard him cry out upon the
name of God; and who's in there instead of him, and
why it stays there, is a thing that cries to Heaven [...].'"  Poole
imagines that whoever hurt his employer is still in the room, though he cannot imagine why such
a person would stay.  He says that for the last week, this person or "'whatever it is that
lives...