Zaroff explains what he does in order to keep his interest in hunting. What is the irony in Rainsford's reaction based on his initial conversation with Whitney on the ship?

Zaroff cites that he has kept his interest in hunting by inventing new prey. Rainsford is appalled. This reaction is ironic because Rainsford had earlier spoken to the fact animals have no feelings or comprehension of human emotions..... at least until he finds out that Zaroff's new animal is the "human animal".