General Zaroff is uncivilized because he lures unsuspecting ships to his island with lights that are supposed to indicate safe passage, but do not. Like the Sirens in The Oddysey, Zaroff lures sailors to their doom. The ships are destroyed on rocks, and the men are at the mercy of Zaroff. He treats them well at first, offering them expensive food and drink in his mansion, but the illusion of civilization ends there. Zaroff is merely attempting to physically strengthen his prey to make them better to hunt. He believes that there are certain people in this world who are expendable, sailors among them, and that hunting them does no harm to the "civilized" world and may even be considered a service to the rest of humanity.
Zaroff is uncivilized in his actions because he has chosen to create this kind of fantasy island where the common rules of society cease to exist. He traps men and then he hunts them down and kills them. This certainly is a game which supersedes any type of civility in any society. What is ironic about his game is that he truly believes the game to be fair and somewhat civilized as well. He doesn't quite understand Rainsford incredulity about it. In fact, he thought that if anyone would understand the desire for more cunning game which can think and reason it would have been a world renowned hunter like Rainsford. He sees it as fair and civilized because he has rules put into place. The hunted also receives a head start and supplies to use, which he believes makes this more a game than a hunt. If the hunted can elude death for three days, then the hunted may go free. Up until the time he meets Rainsford however, he has yet to lose his little game. The barbarism with which Zaroff approaches hunting by hunting humans completely escapes. He feels a sense of entitlement to what he does because he can't seem to find game to hunt that excites him any longer.
It's funny how ironically Zaroff continues to mention his level of civility throughout the story. He appreciates the best food, fine clothes, and many of the amenities only the very weathy can afford. Then uses this power and influence to hunt other human beings. The whole idea of being civil (or civilized) deals with protecting the well-being of others. Zaroff's hunts certainly don't do that.