Why does Rainsford agree to be hunted?

Sangor Rainsford only agrees to be hunted because if he does not, he will be turned over to Ivan, who will undoubtedly torture and kill him.
While General Zaroff talks with Rainsford at dinner, he mentions the lack of enthusiasm for hunting that he had until he created his "dangerous game." But now, he complains, this "ennui" [boredom] seems to be returning because the hunting the night before was not exciting. 

"The fellow lost his head. He made a straight trail that offered no problems at all. . . . It's most annoying."

Since Rainsford recalls having heard the pistol shot, he is well aware of the outcome of that hunt. So, when Zaroff suggests that he and Rainsford hunt that night, Rainsford shakes his head in dissent. "No, general, . . . I will not hunt." Zaroff shrugs his shoulders and replies that the choice is Rainsford's, adding,

"But may I not venture to suggest that you will find my idea of sport more diverting than Ivan's."

Rainsford recalls the general's description of Ivan as once having served as "official knouter" to the Great White Czar." (He issued whippings to criminals.) According to Zaroff, Ivan "has his own ideas of sport." General Zaroff also has mentioned that the others who have been on the island have invariably chosen the hunt. So, since not agreeing to the hunt as suggested by Zaroff means a prolonged and brutal death, Rainsford decides to be hunted by Zaroff. As an accomplished hunter himself, Rainsford figures that he may be able to outwit Zaroff.