In The Most Dangerous Game, why couldn't Zaroff see the difference between killing humans and killing animals?
Essentially Zaroff doesn't view the humans he hunts as humans. His view of them is like the masters of slaves in the South -- the slaves were property, didn't have feelings, couldn't think for themselves, needed to be "taught lessons," etc... Zaroff feels that he is superior to those who he hunts. He doesn't feel that people who shipwreck on his island on his equal. The hunted prove that they are not his equal by not matching his finesse in hunting. They run in straight lines, leave easy-to-follow tracks, etc... Since they are not his equal, they are put on the same level as the animals he had hunted. Rainsford was the only one he ever encountered who gave him any sort of challenge and could equal him in skills and ability. He had also been published and was a very well-known author and hunter. Until then, he just viewed both animals and humans as prey.
He is so engrossed in the hunt that he doesn't see his prey specifically as a human, just a challenge that he must overcome and kill. Those he hunted didn't have "feelings" so to speak. They were merely viewed as the weak being overridden by the strong, not humans hunting humans, the act of barbaric murder.