Zaroff does not give his human prey a fair chance. He has all of the advantages in the hunt.
Zaroff tells Rainsford that he was such a good hunter that he got bored. Hunting even the biggest animals did not challenge him anymore. He was rich enough and arrogant enough that he decided it was his right to set himself up on an island and kidnap men to kill.
First of all, the men do not get a fair chance even in how they end up on the island. Ship-trap Island is surrounded by a dangerous reef, but Zaroff installs a light that indicates that it is safe to pass by it in order to literally trap ships and men. He then kidnaps the shipwrecked men, takes them prisoner, and forces them to take part in the hunt as his prey. Zaroff shows Rainsford the blinking lights.
The general chuckled. "They indicate a channel," he said, "where there's none; giant rocks with razor edges crouch like a sea monster with wide-open jaws. They can crush a ship as easily as I crush this nut."
Once the men are captured, Zaroff offers them a choice to hunt with him or be beaten to death by his assistant, Ivan. Most men take the offer of playing the game, because at least they figure it gives them a chance at escape rather than certain death.
However, it is a very small chance. Zaroff comments that it is not a very high quality class of men that he has on the island, and he has to prepare them for the hunt by keeping them prisoner.
… I treat these visitors with every consideration. They get plenty of good food and exercise. They get into splendid physical condition. You shall see for yourself tomorrow."
"What do you mean?"
"We'll visit my training school," smiled the general. "It's in the cellar. I have about a dozen pupils down there now. …”
During the hunt, the men are also at a disadvantage. Rainsford is given a knife, but Zaroff has a gun. He also has Ivan to assist him, and a pack of trained hunting dogs to track for him. He knows the island much better than any of the men he is hunting, of course. Although Zaroff says that he wants a challenge, he also makes sure that the deck is stacked in his favor.
Zaroff’s downfall was in choosing Rainsford as prey. A trained hunter, Rainsford is also extremely adept at setting traps. It is his ability to think outside the box that wins the game for him though. He not only evades Zaroff, he is able to make it back to his house and kill him. Rainsford realized that the general was playing with him, prolonging the game when he probably could have easily taken Rainsford more than once. He either does not believe that Zaroff will set him free, or he wants revenge for the misery that Zaroff put him through. Either way, he ends the game on his terms by killing General Zaroff.