You might want to focus on how even before Rainsford arrives at the island, the author makes every effort to create a foreboding atmosphere and foreshadow the terrible events that occur on the island. One way he does this is to discuss the sailors' superstitions and worries about the island itself. Note how the author achieves this:
Yes, even that tough-minded old Swede, who'd go up to the devil himself and ask him for a light. Those fishy blue eyes held a look I never saw there before. All I could get out of him was: "This place has an evil name among seafaring men, sir." Then he said to me, very gravely: "Don't you feel anything?"--as if the air about us was actually poisonous. Now, you mustn't laugh when I tell you this--I did feel something like a sudden chill.
Of course, Rainsford is dismissive of such "supersition," but it is important to note that even the toughest of sailors shares these views about the dangerous nature of the island. A clear hyperbole is used to establish the character of the "tough-minded old Swede." By saying that he would "go up to the devil himself and ask for a light" the author establishes his bravery. Clearly, for him to be scared about the island creates a real foreboding of what is actually on that island to be scared about. The strange, almost supernatural chill that the speaker himself feels supports this sense of imminent danger. Of course, when Rainsford is knocked overboard and makes for this island, we realise that we will find out for ourselves what actually goes on there...