Is the competition between Sanger Rainsford and General Zaroff fair?

No, the competition between General Zaroff and Sanger Rainsford is not fair, except in the final scene. 

What is ironic about this situation is that previous to his accidental arrival on Ship-Trap Island, Rainsford has disclaimed any value in his friend Whitney's concern for the prey that they planned to hunt, saying, "Who cares how a jaguar feels?" Now, however, he is in the place of this prey as he is hunted by Zaroff in...

No, the competition between General Zaroff and Sanger Rainsford is not fair, except in the final scene. 

What is ironic about this situation is that previous to his accidental arrival on Ship-Trap Island, Rainsford has disclaimed any value in his friend Whitney's concern for the prey that they planned to hunt, saying, "Who cares how a jaguar feels?" Now, however, he is in the place of this prey as he is hunted by Zaroff in the most dangerous of games in which man hunts man. The game is unfair because Zaroff has weapons and dogs while Rainsford is left his wits only.

However, because Rainsford is a superior and experienced hunter himself, he is able to avoid death, using his knowledge and expertise in laying traps and covering his tracks. Nevertheless, he learns what it is to be "a beast at bay" and his attitude toward the prey of a hunt is greatly altered. In the end, he reverses the roles between him and Zaroff as he aggressively invades the general's bedroom. When the general discovers him, he tells Rainsford,

"I congratulate you....You have won the game."
Rainsford did not smile. "I am still a beast at bay," he said. "Get ready, General Zaroff."
"Splendid! One of us is to furnish a repast for the hounds. The other will sleep in this excellent bed. On guard, Rainsford."

It is only at this point in which the two men duel that the competition is fair. After he wins, Rainsford is again the hunter.