Rainsford and Zaroff both enjoy hunting and are good at it, but Zaroff is bored with hunting animals.
Rainsford is a skilled hunter. He is so experienced and well-respected, in fact, that he has written many books on the subject. However, he still seems to enjoy hunting animals and find a challenge in it. Zaroff, on the other hand, believes that he has no challenge left in animals. He has moved on to try to find animals who can outsmart him—human prey.
Rainsford believes that animals cannot think. He has a conversation about this at the beginning of the story. However, it is clear from this conversation with Whitney that Rainsford still gets pleasure from hunting regular animals.
"We should make it in a few days. I hope the jaguar guns have come from Purdey's. We should have some good hunting up the Amazon. Great sport, hunting."
"The best sport in the world," agreed Rainsford.
"For the hunter," amended Whitney. "Not for the jaguar."
Rainsford does not care how the animals feel, but he does still seem to enjoy hunting something as simple as a jaguar. Even though he is talented enough to have written books on hunting, he can still take pleasure in the hunt. Zaroff, on the other hand, apparently has decided that hunting an animal is beneath him.
When Zaroff explains why he “invented” a new quarry for hunting (humans), he tells Rainsford over and over again that he is one of the best hunters in the world and he simply could not stand to be bored. He tells Rainsford that he would simply “go to pieces” if he did not do something.
"[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."
Zaroff’s plan was to get an island and populate it with human prey. Away from the prying eyes of civilization, he could pretty much do what he wished. Hunting was now much more interesting. Of course Rainsford is the most dangerous game of all. He is a trained hunter. Zaroff knew he would have a lot of fun chasing him and trying to hunt him down.
Rainsford is completely opposed to the hunting of humans. When Zaroff tries to suggest that because he has been a solider he will be okay with killing people for sport, he puts him in his place.
I refuse to believe that so modern and civilized a young man as you seem to be harbors romantic ideas about the value of human life. Surely your experiences in the war--"
"Did not make me condone cold-blooded murder," finished Rainsford stiffly.
Of course Rainsford will not play the game, and hunt humans alongside Zaroff. The general is completely okay with that. He would much rather hunt Rainsford anyway, because, as I said, Rainsford is an excellent hunter and will make more challenging prey for Zaroff. To Zaroff, it is all about making the game more challenging.
Rainsford is an above-average hunter but a typical human being. He considers animals killable and balks at killing people. Zaroff, on the other hand, follows his own moral code. He is completely narcissistic. If it pleases him, it is moral. He considers himself superior to others because he survives. In his world, it is kill or be kill. If you survive, you are worthy. If you don’t, you are not. In the end, though, it is Rainsford who survives.