Answer:
The goal of the porter is to serve as an alert to the reader that something terrible is going to happen in the castle.
Step-by-step explanation:
When the porter refers to hell, he warns the reader that something big and so terrible is going to happen that can be compared to hell and all the wickedness and agony that this place symbolizes. This situation is the murder of the king in cold blood and the ususpation and persecution that the murderer will provoke, as well as all the mental lack of control and emotional exhaustion that will be presented during the narrative.