General Zaroff is not a monster in a literal sense. He does not have fangs, he is not paranormal, etc... His actions, on the other hand, could be considered by many as monstrous. He hunts humans. He tortures innocent people by trapping them on the island, and then hunting them down. All of this he does for the sole purpose of sport. There's no reason for it whatsoever except for the fact that he's bored with the traditional "animal" game. If the people he traps do not want to participate, he gives them to Ivan to destroy. In essence, he's a monster because of the way he treats other living things.
General zaroff is a monster. He is a monster because of his craving for blood.