General Zaroff is the antagonist in this story, so most character traits that are associated with him are not positive. I'd like to go a different route with this question, though. I would like to focus on Zaroff's positive characteristics. It's how Zaroff twists those positives that make him such a great antagonist.
First, General Zaroff is a calm and not easily flustered man. He puts his own life at risk when hunting animals, and his life is even more at risk when he hunts humans; however, he is completely calm when on the hunt. Even at the end of the story when Rainsford jumps off of the cliff, Zaroff essentially shrugs and moves on.
When the general and his pack reached the place by the sea, the Cossack stopped. For some minutes he stood regarding the blue-green expanse of water. He shrugged his shoulders. Then be sat down, took a drink of brandy from a silver flask, lit a cigarette, and hummed a bit from Madame Butterfly.
That calmness is also displayed in his conversations with Rainsford. Rainsford flat-out calls Zaroff a murderer, but Zaroff doesn't even bat an eye at the accusation. When a reader looks at the descriptions of the conversation, Rainsford is saying things "hotly" or "stiffly." He's agitated and angry. Zaroff, on the other hand, doesn't get any colorful descriptions. His quotes end with "he answered," "he said," etc. He's utterly calm. He's even calm right before Rainsford kills him.
The general made one of his deepest bows. "I see," he said. "Splendid! One of us is to furnish a repast for the hounds. The other will sleep in this very excellent bed. On guard, Rainsford."
Zaroff is creative. Granted, his creativity is morally suspect, but he does come up with a creative solution to his problem. Zaroff is bored with hunting regular big game. He decides that he needs a form of prey that doesn't rely only on instinct. He needs his prey to be able to reason, so he decides that humans will be his new prey. He then implements an effective plan to lure human prey to his island. Essentially, Zaroff has figured out a way to stock his island with prey.
Zaroff is intelligent and experienced. Rainsford comes up with all kinds of different things to throw Zaroff off of his trail. Rainsford even sets up traps, but none of it works. Zaroff is too knowledgeable about hunting to be fooled. Zaroff is also well-read. Zaroff admits that he has read Rainsford's book. The comment is made in such a way that it gives readers the impression that Zaroff has read many other books too. Additionally, Zaroff takes Rainsford to his library and not his trophy room.
"And now," said the general, "I want to show you my new collection of heads. Will you come with me to the library?"
In a twisted way, some of Zaroff's actions give the superficial appearance of being kind. For instance, he gives his human prey the opportunity to be as physically fit as possible before the hunt--but only because he wants the hunt to be a challenge.