With these two, we have the younger hunter meeting an older, more experienced and cynical version of himself. So as foils tend to be: they are opposites even as they share some innate qualities. They are versions of the same self, separated—so they think—by ideology. But their similarities are greater than their differences, as is confirmed all too dramatically at the story’s end. Let’s just put the hunting thing aside for a second and consider what else these chaps have in common.
We know that Zaroff is a wealthy fallen aristocrat who enjoys all the best that civilization has to offer; he is sure to talk about and display all of the evidence of his high-rolling lifestyle. (The guy has champagne shipped in, for crying out loud).
But what about Rainsford? Well, he’s not exactly one of Zaroff’s cast-offs. The fact that Rainsford is of a higher class than Zaroff's usual quarry indicates that Zaroff is making an exception. But then he technically can’t justify hunting him because he is not “scum of the earth.” But why quibble over details?
Rainsford is well-off enough to be touring the world hunting big game and traveling in a luxury yacht (not exactly Zaroff’s usual “tramp ship”). Once he is at Zaroff’s haunted house, Rainsford admires everything, from his borrowed evening suit, which he identifies as coming “from a London tailor who ordinarily cut and sewed for none below the rank of duke” (oh, and it fits—so they are roughly the same size), to “the table appointments [which] were of the finest—the linen, the crystal, the silver.” It’s doubtful that Zaroff's usual mongrel-predator enjoys filet mignon with him the night before the hunt.
These two just belong together.