The description of General Zaroff’s smile foreshadows his violent and savage nature.

Foreshadowing is a hint by the author of what’s to come.  Authors drop these little clues early on to build suspense and create a mood.

When Rainsford first meets General Zaroff, he does not know anything about him.  Rainsford has published books on hunting, so Zaroff knows much more about him than he does about the general.  The general is thrilled to find a skilled hunter in his clutches.

Although the description of the house is very grand and genteel, the description of the general hints at his dark side.  The general introduces Ivan, the large mute brute, and Rainsford is definitely disturbed.

"He is a Cossack," said the general, and his smile showed red lips and pointed teeth. "So am I."

The red lips will make the reader think of blood, and the pointy teeth seem like an animal’s.  This is ironic, because in the story Rainsford is the prey and Zaroff is the hunter.  By this mere description, the reader begins to wonder if Zaroff is dangerous, and feel unsettled by him.

The use of foreshadowing in this story is not the only form of suspense.  The classic predator and prey relationship is explored in different ways from beginning to end, starting with the conversation on the boat about whether or not animals can think.

What details in Zaroff's description foreshadow his true nature?

When Rainsford initially arrives at General Zaroff's impressive chateau, Zaroff descends the steps and immediately recognizes his guest as a renowned hunter. Rainsford's first impression of General Zaroff is that he is "singularly handsome." However, Rainsford also notices that there is some bizarre quality about the general's face. General Zaroff is then described as being a tall man with vivid white hair and a black pointy mustache. He is also described as having the face of an aristocrat, with very black and bright eyes. Rainsford also describes Zaroff's face as being dark and resembling a man used to giving orders. Judging from Rainsford's description, the bizarre, dark, and resolute appearance of General Zaroff's face foreshadows his twisted, maniacal nature. The words "bizarre," "dark," and "black as the night" are ominous and reflect General Zaroff's deranged personality. When Rainsford finally dines with the general, he discovers that Zaroff hunts humans on the island in order to satisfy his desire to hunt competitive prey.

How do Zaroff's physical features warn about his brutal nature in "The Most Dangerous Game"?

Before readers meet Zaroff, his cruelty is indicated by his choice to hunt with a .22 instead of a larger gun, meaning he will need more shots to take down his prey. His house is stationed like a military fort, which is also a foreboding sign.

When readers are introduced to General Zaroff, they meet a man who looks aristocratic, with his officer's uniform, his finely-waxed mustache, and his pointy but handsome features. This visible connection to wealth has some implications of a connection to brutality. He has beady, black eyes, which are associated with evil. He also smokes long, black cigarettes, that seem mysterious and foreboding. His body language, laughing so heartily while he talks of hunting, is also a strong indicator that he is building towards a not-so-pleasant conclusion.

What details about Zaroff's appearance foreshadow his true nature in "The Most Dangerous Game"?

Let's look at the details exactly as Connell writes them so we can uncover words that might foreshadow the truth about his nature, which we know to be a cold-hearted killer who kills for sport, and to relieve himself from boredom:

In a cultivated voice marked by a slight accent that gave it added precision and deliberateness, he said, "It is a very great pleasure and honor to welcome Mr. Sanger Rainsford, the celebrated hunter, to my home."

... 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.

There are many descriptors in here which echo the necessities of a highly successful hunter. Look at the words I have italicized for you.

Rainsford also notes that this man seems bizarre. It is almost as if Rainsford wonders if this man were capable of great strange things. Look at the bold. We believe we can see much in the way of a person's motive and morals by looking them in the face.

To add, we also look to the eyes. Closer looks at his face reveal great black and dark features. Look at what I have bolded and italicized for you. Dark and black colors are mentioned 3 times. This signifies evil.

Finally, the comments about him as in control of himself, a military figure, and one who commands others further demonstrate his control of the situations in which he partakes.

How do Zaroff's physical features warn about his brutal nature in "The Most Dangerous Game"?

The text does give readers some clues as to Zaroff's scrofulous nature. The earliest hint that I can find is when he extends his hand in greeting to Rainsford. Readers are told that Zaroff (whose name we don't know yet) has a slight accent. In my film studies class, we studied heroes and villains at one point, and a key motif (which has its roots in racism) in modern Western storytelling is to make villains dark, foreign, and/or ugly. Zaroff's accent points to his foreignness, and this often serves to trigger a reader's suspicion of a character—as readers have been trained to perceive these cues as negative or suspicious.

Rainsford then gets a decent look at Zaroff, and Rainsford's early impression is that there is something "bizarre" about the man's face. This falls in line with the foreignness impression. There is definitely something different and off-putting about Zaroff. We are also told that Zaroff's eyebrows, mustache, and eyes are black. That circles back to the "dark" physicality of villains (think about the imagery of Darth Vader or Lord Voldemort). All of these clues about Zaroff being dark and foreign-looking should hint that the man is a villain.