Question

In Richard Connell’s short story “The Most Dangerous Game,” Zaroff’s aristocratic appearance and dignified manners mask his sinister, animalistic nature. Which line from the excerpt best provides a glimpse into his true nature? Rainsford’s first impression was that the man was singularly handsome; his second was that there was an original, almost bizarre quality about the general’s face. He was a tall man past middle age, for his hair was a vivid white; but his thick eyebrows and pointed military mustache were as black as the night from which Rainsford had come. His eyes, too, were black and very bright. He had high cheekbones, a sharpcut nose, a spare, dark face—the face of a man used to giving orders, the face of an aristocrat. Turning to the giant in uniform, the general made a sign. The giant put away his pistol, saluted, withdrew. "Ivan is an incredibly strong fellow," remarked the general, "but he has the misfortune to be deaf and dumb. A simple fellow, but, I'm afraid, like all his race, a bit of a savage." "Is he Russian?" “He is a Cossack,” said the general, and his smile showed red lips and pointed teeth. “So am I.” … Half apologetically General Zaroff said, “We do our best to preserve the amenities of civilization here. Please forgive any lapses. We are well off the beaten track, you know.” … Laughter shook the general. “How extraordinarily droll you are!” he said. “One does not expect nowadays to find a young man of the educated class, even in America, with such a naïve, and, if I may say so, mid-Victorian point of view. It’s like finding a snuffbox in a limousine. Ah, well, doubtless you had Puritan ancestors.

Answer 1

"and his smile showed red lips and pointed teeth." is a good hint.

If you think about it, this line is the only one that would be used to describe an animal. 
You can ask yourself, what do you think of when you read that line. 
When I read it I could almost imagine a predator like a wolf snarling, showing it's vicious teeth. In effect, when he opened his mouth to smile it's like you're getting a glimpse into him and his nature.