What happens at the end of the story? Why does Rainsford say "I am still a beast at bay"?

Rainsford is cornered by Zaroff and his remaining dogs. Rainsford jumps off a cliff into the ocean. Zaroff presumes Rainsford is dead. Rainsford however is a perfect man-hero and swims back to the mansion. There he waits for Zaroff in his bedroom in Zaroff's pyjamas (kind of creepy) they fight and Rainsford of course kills him. Rainsford meant that Zaroff had not yet caught his animal yet (hence beast at bay) signalling their final confrontation.