What descriptions about Zaroff's physical features serve to warn Rainsford (and the reader) about the man's brutal nature?

Zaroff is described through the eyes of Rainsford, the narrator of the story, and also through his own dialogue and description. 

Though Rainsford's first impression of Zaroff is that he is "singularly handsome," he remarks in the same phrase that there is an "original, almost bizarre quality about the general's face," which is a more negative description. Later, Zaroff declares himself "a bit of a savage" because he is a Cossack, (a person from southern Russia/Ukraine). Zaroff is later revealed to have "pointed teeth," which are ominous characteristics. Though the general starts off as a polite and gracious host, Rainsford is unnerved by the general's constant appraising stares. The general's "red-lipped smile" is also unnerving. It's clear that he is a brilliant and dangerous man from his conversations about hunting. 

Even the general's actions highlight his brutal nature. When discussing the hunting of men, the general chuckles and laughs repeatedly. He drops a walnut on the floor and crushes it with his heel to emphasize a point. Ultimately, he challenges Rainsford to the hunting game, which demonstrates his cruelty and brutality in his willingness to hunt a fellow man.