In the conversation that Rainsford and Zaroff have together before the beginning of the "most dangerous game," Zaroff introduces this viewpoint and attempts to justify it to Rainsford, who is understandably shocked by Zaroff's idea. Zaroff is rather amused by Rainsford's scruples and the way that he "harbours romantic ideas about the value of human life." What to Rainsford is simply "murder" to Zaroff can be justified by looking at life from a Darwinian perspective. Note how he attempts to persuade Rainsford:
Life is for the strong, to be lived by the strong, and if need be, taken by the strong. The weak of het 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? I hunt the scum of the earth--sailors from tramp ships--lascards, blacks, Chinese, whites, mongrels--a thoroughbred horse or hound is worth more than a score of them.
Thus Zaroff looks on the world and life as an arena where those who are stronger and more powerful have every right to use and abuse those who are less powerful than them in any way they like. Thus, if you side with this view, you can see how his pursuing men is justified. In addition, according to Zaroff, he hunts men because of their characteristics and how they have "courage, cunning, and above all, it must be able to reason." Man's ability to reason makes it a prime quarry for Zaroff, who on his island, far from civilisation, is able to create his own moral code and follow his own laws.