Zaroff tells Rainsford he is bored and the humans he is hunting are inferior, which is how they end up killed.
Zaroff is an expert hunter. He explains to Rainsford that he became so good at hunting that he could hunt any animal of any size and it was no longer a challenge. Perfection was a bore.
No animal had a chance with me any more. That is no boast; it is a mathematical certainty. The animal had nothing but his legs and his instinct. Instinct is no match for reason. When I thought of this it was a tragic moment for me, I can tell you.
This is the reason, he told Rainsford, that he developed a new type of game. Rainsford was confused by this. He was an avid hunter, too. He wrote books on the subject. He knew Zaroff could not just create a new animal.
Zaroff explains he bought the island because it was perfect for this new type of game hunting due to its “jungles with a maze of traits in them, hills, swamps.” Zaroff says his new animal can match wits with him, and he is never bored. In Zaroff’s mind, that is also a justification because he is extremely egotistical and it is all about him.
Zaroff justifies hunting people because he is trapping ships and taking sailors. If he kills them, he believes that means they are weak and deserve to die. He uses the survival of the fittest argument, saying,
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? If I wish to hunt, why should I not? I hunt the scum of the earth.
There is a certain amount of racism here, too, since Zaroff claims that the "lassars, blacks, Chinese, whites, mongrels" are worthless human beings. Because of their profession, class, and/ or race, they are thought by Zaroff to be inferior to him, meaning he can do what he wants with them. He is an aristocrat. He has money and power. They do not.
What Zaroff fails to note in his argument is that he has all the advantages in the hunt. He knows the island. He also has a gun, and all he gives the person he is hunting is "a supply of food and an excellent hunting knife" and three hours' head start. It doesn't negate his advantage.