Zaroff's decision to hunt human beings comes from his general boredom from killing wild animals.
"I exhausted their possibilities, you see. No thrill left in tigers, no real danger."
Although he had killed more animals than he could remember, they no longer were a challenge for the Cossack. Neither the cape buffalo, which Rainsford called the most dangerous of all animals, nor the jaguar, for which Zaroff traveled to the Amazon to hunt, interested him anymore. He worried that he would "go to pieces" without his greatest love. They no longer fascinated him.
"Simply this: hunting had ceased to be what you call 'a sporting proposition.' It had become too easy. I always got my quarry. Always. There is no greater bore than perfection."
The animal's instinct was no match for human reasoning, Zaroff told Rainsford. This tragic realization was the inspiration for a new type of hunt with a different type of prey.
Zaroff believed that
"Life is for the strong, to be lived by the strong, and, if needs be, taken by the strong. The weak of the world were put here to give the strong pleasure. I am strong. Why should I not use my gift?"
Zaroff, a nobleman, did not view humans as equals. He believed he had the right to hunt, in what he considered a civilized manner,
"... the scum of the earth: sailors from tramp ships--lassars, blacks, Chinese, whites, mongrels--a thoroughbred horse or hound is worth more than a score of them."
"But they are men," said Rainsford hotly.
"Precisely," said the general. "That is why I use them. It gives me pleasure. They can reason, after a fashion. So they are dangerous."