General Zaroff had three unfair advantages.
First, Zaroff wanted to hunt humans and he had been doing so, even if we do not know for long long he has been doing this. On the other hand, Rainsford does not want to be hunted. In other words, he is an unwilling participant. Rainsford even says that he wanted to leave immediately.
Second, Zaroff had help whereas Rainsford was alone. For example, Zaroff had Ivan, who is described a huge. Moreover, he had the use of his dogs, which gave a greater unfair advantage.
Finally, Zaroff had the benefit of knowing the island better than Rainsford. In fact, Rainsford knew nothing of the place. He just arrived there. In hunting or any combat situation knowing the terrain is a huge advantage.
In conclusion, the whole hunt was slanted to favor Zaroff. That Rainsford won is a testament to his skill as a hunter.