How does Zaroff justify his hunting of humans?
Basically, Zaroff justifies his hunting of humans with the old saying "might makes right." It is acceptable for him to hunt human beings because he is strong enough, powerful enough to do it and get away with it. As he says
"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 goes on to argue that the kinds of people he hunts are really inferior. He says that a hound is worth more than 20 of these kind of people. So what he is saying is that he and others like him are sort of a natural nobility. They were born superior and therefore they have the right to do pretty much whatever they want to do to the inferior people.
Zaroff is a hunter whose only passion is seeking and eliminating the prey. But, he is becoming bored with his constant success in hunting animals. They do not provide a challenge that stimulates him. Since his life would have no purpose if hunting holds no challenge, he turns to an animal that can reason-man. At first he hunts only the shipwrecked sailors that survive merely to become prey on his island. Sailors, to him, are low life but provide more challenge than four-legged beasts. When Sanger Rainsford finds refuge on his island, Zaroff finds a challenge more worthy of his skills. After three opportunities to kill the stranded hunter, Zaroff means to finish him off. He has been amused and intrigued by Rainsford's skills. To him, the life and death struggle is merely a game, the game for which he lives.