Right from the beginning the tone is mysterious and ominous. Here is how the story begins:
"OFF THERE to the right--somewhere--is a large island," said Whitney." It's rather a mystery--" "What island is it?" Rainsford asked. "The old charts call it `Ship-Trap Island,"' Whitney replied."
Just from here, we have the words, "somewhere," "mystery," and "Ship-Trap." This says that they are in an exotic location where danger lurks.
The topic of hunting also exacerbates the mystery and danger. Here is what Rainsford says:
Be a realist. The world is made up of two classes--the hunters and the huntees. Luckily, you and I are hunters. Do you think we've passed that island yet?"
Without Rainsford knowing it, he will become a huntee.
There is also the odd meeting with General Zaroff. Undoubtedly, Zaroff comes off as a sophisticated man who is educated and a good host, but there is something sinister about him.
"Not in the least," declared Rainsford. He was finding the general a most thoughtful and affable host, a true cosmopolite. But there was one small trait of .the general's that made Rainsford uncomfortable. Whenever he looked up from his plate he found the general studying him, appraising him narrowly.
When the story comes to full swing, all of Rainsford's fears are confirmed. Zaroff is a madman.