What was the ‘most dangerous game’ that General Zarrof loved to play? Why did Rainsford participate in such ‘game’?

The actual game involves Zaroff hunting people on his private island. Zaroff offers Rainsford the chance to hunt with him but Rainsford draws the line at hunting people. Zaroff had no choice but to become the prey. If he refused, Ivan would kill him. Rainsford takes Zaroff's wager.