Why has Zaroff begun hunting human “game”

Zaroff begins hunting human "game" because hunting animal game has become boring to him. He feels it is no longer hold any sort of challenge for him. He feels that he can outsmart any animal- even the most cunning, which is why he has switched to human game. Humans can think and reason which makes them more interesting to hunt. Since Zaroff is civilized, rich, cultured, well traveled, and the like he feels that he is completely justified in his game- after-all it is fair, or so he thinks, because he gives his game a fighting chance.