As the question stated, this is an opinion based question. The author does not explicitly tell readers what Rainsford did and did not learn from his experience with Zaroff. The answer could go a few different ways.
1. Rainsford learned nothing. Rainsford is an experienced hunter that tells his colleague in the beginning of the story that prey animals have no feelings. They do not experience fear or anything similar. As prey, Rainsford felt fear, but he still doesn't believe that animals experience the same feeling. He believes this because he still considers humans different from the prey species that he normally hunts. Readers know that Rainsford considers humans different because he admits that he believes humans are the only animals capable of reasoning. Despite his experience with Zaroff, Rainsford still feels that humans are different from animals, and animals experience no fear.
2. Rainsford now has learned that the animals that he hunts experience fear. The fact is interesting to Rainsford, but he still continues to be an avid hunter; however, he now has more respect and compassion for the animals that he hunts and kills.
3. Rainsford learns that hunted animals experience fear. Because of this knowledge, Rainsford gives up hunting completely.
4. Rainsford is so thrilled from his experience with Zaroff that he decides he wants more of the experience. He has learned that Zaroff was correct and the ultimate hunting thrill is hunting humans. Rainsford takes over the island and continues to use it in the same way that Zaroff used the island. Rainsford now becomes a hunter of men.