Zaroff knew who Rainsford was because he was a famous author of hunting books.
Sanger Rainsford is famous in the hunting world, and General Zaroff is a serious hunter. He is so passionate about hunting that there probably isn’t a book about hunting that he hasn’t read. When Rainsford falls off of his ship and swims to the spooky Ship-trap Island, he doesn’t expect to find it inhabited. However, there it is: a huge chateau that he first thinks is a mirage. It is the humble abode of General Zaroff.
Rainsford tries to convince Ivan, the mute giant, that he isn’t a criminal breaking in. Zaroff knows this though.
"Perhaps," said General Zaroff, "you were surprised that I recognized your name. You see, I read all books on hunting published in English, French, and Russian. I have but one passion in my life, Mr. Rainsford, and it is the hunt."
Rainsford is not impressed by this, but he does seem impressed by Zaroff’s collection of heads. He comments that the Cape buffalo is the “most dangerous” game, and Zaroff corrects him. He says he has more dangerous game on the island. I always imagine at this point when I read the story that he might be thinking to himself that he is looking at it!
Zaroff invites Rainsford to dinner, and soon, Rainsford, the seasoned hunter, finds out what the most dangerous game is.
"But no animal can reason," objected Rainsford.
"My dear fellow," said the general, "there is one that can."
"I can't believe you are serious, General Zaroff. This is a grisly joke."
"Why should I not be serious? I am speaking of hunting."
"Hunting? Great Guns, General Zaroff, what you speak of is murder."
Since Rainsford is not up to the challenge of hunting men, because his conscience does not agree, he ends up being prey instead of predator. Rainsford, as one of the best hunters in the world, makes very good prey. He ends up winning the game. One of the first things he does when he wins is change the rules.
Rainsford did not smile. "I am still a beast at bay," he said, in a low, hoarse voice. "Get ready, General Zaroff."
Zaroff is going to let him go, presumably, but Rainsford has other ideas. He is angry at Zaroff for making him play the game, and turning him into prey. He lets Zaroff turn him into the one thing he didn’t want to become—a murderer.
General Zaroff is a sociopath. He thinks that since he is alone on this island far from civilization that he can make his own rules. To him, there is only the weak and the strong. He is strong, so the weak have no value. There is no innate value in human life. One of the lessons of this story is that morality is not a constraint of society. It is something we carry with us. Whatever the challenge, we have to somehow hold on to our values and morals. Without them, we lose our humanity and self.