Sanger Rainsford finds himself in a terrible predicament in Richard Connell's "The Most Dangerous Game." He can be tortured until he is eventually killed by Ivan, the deaf-mute Cossack, or he can become General Zaroff's prey and fight for his life. Neither choice is a good one and both will probably end in his death, but Rainsford chooses being hunted.
As he does everything else, General Zaroff sets the terms of the hunt in the most civilized of terms: if, by midnight of the third day, Rainsford is able to elude capture (and death), he will be allowed to leave the island. Rainsford is given some supplies and has a few hours head start on Zaroff, who prefers to hunt at night.
On his first day of being hunted, Rainsford decides to create an intricate trail, something he does not believe the general will be able to follow, especially int he dark. Unfortunately, Rainsford is wrong.
Rainsford ends up in a tree, praying that as the general gets closer he will not see Rainsford. Soon, however, Zaroff is upon him, and Zaroff's eyes "were traveling inch by inch up the tree." Rainsford, of course, is frozen with fear and knows this is probably the end for him. Instead, the general's eyes
stopped before they reached the limb where Rainsford lay; a smile spread over his brown face. Very deliberately he blew a smoke ring into the air; then he turned his back on the tree and walked carelessly away, back along the trail he had come.
Though Rainsford is relieved, he is also sick with fear for his life to realize that the general is a such a skilled hunter, even in the dark. Rainsford thinks it is luck that kept Zaroff from looking any higher in the tree, but then he has a shattering revelation about the cause of General Zaroff's smile and departure.
Rainsford did not want to believe what his reason told him was true, but the truth was as evident as the sun that had by now pushed through the morning mists. The general was playing with him! The general was saving him for another day's sport! The Cossack was the cat; he was the mouse. Then it was that Rainsford knew the full meaning of terror.
We may not have known why the general smiled, but certainly Rainsford has figured it out. Zaroff smiled because he realizes that hunting Rainsford is going to be the challenge he has been looking for, and he does not want it to end too soon. He smiled because he could have killed his prey right then but decided to keep it (him) alive for another day of challenging hunting.