Rainsford’s experience might make him more ruthless, because he killed general Zaroff when he used to dislike murder.
Rainsford was an experienced hunter before his experience on the island. He had written many books on the subject, and had travelled the world. However, he drew the line at killing people. He considered killing people immoral. We can tell this from his reaction when he finds out that his host and captor General Zaroff kidnaps sailors and other men and makes them play a game where they are prey and he is the hunter.
This conversation between Rainsford and Zaroff demonstrates Rainsford’s views on the subject. See how he answers Zaroff when he suggests that killing in war is the same thing as killing a man in hunting.
"I refuse to believe that so modern and civilized a young man as you seem to be harbors romantic ideas about the value of human life. Surely your experiences in the war--"
"Did not make me condone cold-blooded murder," finished Rainsford stiffly.
However, when Rainsford is forced to play the game, something funny happens. He does use all of his cunning and wile to evade Zaroff. He is afraid the entire time. He does not like this feeling of fear. He calls it a “cat and mouse” game and, and he does not at all like being the mouse. He also does not like it that the cat is playing with the mouse. Zaroff does not just try to get him, he toys with him. He drags things out. He commends him when he does well, and seems to enjoy the challenge. This really annoys Rainsford.
Rainsford only has to survive for three days, and then Zaroff will let him go. Those are the conditions of the game. However, when Rainsford breaks into Zaroff’s house, he changes the game. When Rainsford comes to see Zaroff, it is not with a mind to end the game. When he describes himself, he does not even seem to use himself as human.
Rainsford did not smile. "I am still a beast at bay," he said, in a low, hoarse voice. "Get ready, General Zaroff."
As the story ends, Rainsford seems to not only have killed Zaroff, but enjoyed it. He did not have to. Zaroff was happy to see him, and happy that he won. Rainsford was not happy until he had killed his tormenter. The experience in the jungle had changed him. He said before that he was not a murderer. Yet he did not need to kill Zaroff. He had won the game. He could have left. Instead, he killed the man. He had changed. He was a murderer. War may not have changed him, but this experience did.