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

From the text:

He was finding the general a most thoughtful and affable host, a true cosmopolite. But there was one small trait of the general's that made Rainsford uncomfortable. Whenever he looked up from his plate he found the general studying him, appraising him narrowly.

A trace of anger was in the general's black eyes, but it was there for but a second; and he said, in his most pleasant manner, "Dear me, what a righteous young man you are!