Saying that life is for the strong, to be lived by the strong, and if need be, taken by the strong isn't justifying your horrible actions. These men are innocent and you're using them for your pleasure.
As a renowned hunter, I would advise you not to be doing these type of activities. It may have a chance of ruining your image in the future and you may be making the wrong choices.
Here, Rainsford is arguing with Zaroff and getting him to sympathize with the people that he's been killing out of boredom. It's helping him to understand that his actions are wrong and there is a better way help you're boredom rather than go off and create murder.
Rainsford is advising Zaroff not to continue on with killing others just for a game and to please himself as a renowned hunter himself. If people find out he's been doing this, it will ruin his image as a hunter and his work will be down the drain.