(I have edited your query since only one question per eNotes post is allowed.)
To Rainsford, this question is quite simple. He is a hunter, not a murderer. Although he has killed men during World War I, Rainsford differentiates between battle conditions and the cold-blooded killing of human prey. Rainsford considers Zaroff's new game uncivilized and "a grisly joke." Zaroff, on the other hand, sees the killing of his human prey as a contest between the strong and the weak.
"Life is for the strong, to be lived by the strong, and, if needs be, taken by the strong. The weak of the world were put here to give the strong pleasure. I am strong. Why should I not use my gift? If I wish to hunt, why should I not?
Zaroff considers the sailors he shipwrecks as the
"scum of the earth... lassars, blacks, Chinese, whites, mongrels--a thoroughbred horse or hound is worth more than a score of them."
Zaroff views Rainsford's "puritan" values as romantic--a remnant of the Victorian era. To him, killing a man during the hunt is merely a game in which both sides have a chance to win--though Zaroff decidely stacks the deck in his favor. By the end of the story, it is unclear whether Rainsford has changed his mind about the barbarity of the act or whether his actions only become that of revenge. Zaroff has not changed, however. He addresses Rainsford with a bow and exclaims "Splendid!", knowing full well he will now become the beast at bay.