General Zaroff knew who Rainsford was because Rainsford had published books on hunting.
When Rainsford falls off the ship and swims to a remote island, he discovers that the island is inhabited by a chateau. The chateau’s owner is an avid hunter who professes to have enormous skill. He is impressed immediately upon hearing Rainsford’s name.
"I've read your book about hunting snow leopards in Tibet, you see," explained the man. "I am General Zaroff."
His feelings about Rainsford do not get him off the hook. It just makes Rainsford more interesting prey. He forces Rainsford to go hunting with him, but not as a fellow hunter. He hunts men because they are more interesting than animals, and a world-class hunter is perfect.