What is the effect of comparing Zaroff and Rainsford to a cat and a mouse?
This is a basic metaphor. Zaroff is the cat in this relationship because he is the predator, stalking Rainsford, his prey. Rainsford is the mouse because he is incredibly vulnerable to Zaroff. Zaroff has many advantages that make victory (Rainsford's death) seemingly inevitable. Firstly, Zaroff is hunting Rainsford on his own island. Rainsford is trapped in a limited physical space and he is unfamiliar with the terrain. Zaroff is clearly at an advantage. Secondly, Zaroff has guns, dogs, and a brute of a servant to assist him as he hunts. Rainsford has a hunting knife, but no other weapons. He must rely only on his brain and ability to improvise. Again, Zaroff has the upperhand. Lastly, Rainsford has never been hunted before, but Zaroff has experience hunting humans; he can anticipate what men are likely to do and be a few steps ahead of them. He enjoys "toying" with his prey, like a cat hunting a mouse. Instead of killing it at the first chance, a cat will prolong the experience for its own enjoyment. Zaroff does this the first day while hunting for Rainsford. After Zaroff correctly tracks Rainsford to a tree, he rests, smokes, and returns home without killing Rainsford. Hunting stimulates him, just as it does for a cat, so he prefers to make it last.