At the ending of this story, it is not clear that there really is a difference between Rainsford and Zaroff. At least, some have said that the ending implies that Rainsford may just step into Zaroff's place and start hunting himself.
However, you can argue that they are different, especially if you don't believe this interpretation of the ending. While Rainsford does like to hunt, he does nothing that shows that he is brutal like Zaroff. Rainsford does set all sorts of traps for Zaroff, but that is completely self-defense. Rainsford kills Zaroff, not in self-defense, but it's hard to imagine what else he should have done -- go to sleep and hope Zaroff wouldn't come kill him?
So, I think that Rainsford is not cruel and inhuman like Zaroff is. Rainsford is only violent when he needs to be.
Zaroff and Rainsford are both hunters. Rainsford is world renowned for his skill as a hunter, but General Zaroff believes that he too is a skilled hunter and he has heard of and is excited to "hunt with" Rainsford. The biggest difference between the two is that Rainsford hunts big game that are only animals, while General Zaroff hunts what he calls "dangerous game" and that is humans. He believes that his "game" where he hunts a human for three days is fair and it intrigues him because he feels he has finally found his intellectual match in a hunt. He feels that only humans are cunning and clever enough to be hunted. Rainsford does not agree, but is forced to play.