This is a good question. Rainsford's impression of the general was mixed. His immediate impression was that the general was a cultivated man. He also knew that he was not an American, because he had an accent. Moreover, Rainsford might have felt a tinge of joy, because the general stated that he read his book on hunting snow leopards. 

As for appearance, Rainsford stated that the general was a handsome middle-aged man. It also appeared that the general was an aristocratic man.

Despite these observations, Rainsford also felt a sense of uneasiness. There was something about Zaroff that was not quite right. In particular, there was something about Zaroff's face that was ominous. What undoubtedly contributed to this feeling was that he had a tall mute, Ivan, next to him with a gun in his hand. Here is an lengthy quote that gives you what Rainsford says:

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."

Automatically Rainsford shook the man's hand.

"I've read your book about hunting snow leopards in Tibet, you see," explained the man. "I am General Zaroff."

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.