I want to talk about four clues that seem important here.
First, you have the fact that Dr. Roylott has moved Miss Helen Stoner into the room that used to be her sister's room. This suggests that there is something in particular about that room that helps him kill people.
Second, you have the fact that Roylott put the bell pull in without Julia asking for it. Why would he do that if it did not have to do with her death?
Third, the bell pull is not attached to a bell -- just to a hook in the vent.
Last, the vent leads to the next room, not to the outside. It would make more sense and be easier for a vent to hook up to the outdoors.
So the vent and bell pull are weird and they are put in this room where Roylott wants Helen to sleep. Very suspicious.