Why does General Zaroff enjoy hunting humans?

From the text:

"I wanted the ideal animal to hunt," explained the general. "So I said, `What are the attributes of an ideal quarry?' And the answer was, of course, `It must have courage, cunning, and, above all, it must be able to reason."'
"But no animal can reason," objected Rainsford.
"My dear fellow," said the general, "there is one that can."