In Richard Connell's "The Most Dangerous Game," whether Rainsford is responsible for Zaroff's death or not can be argued two ways. First, one could argue that Rainsford is responsible for Zaroff's death simply because he wins the final battle. Two men, seemingly of equal strength and wits, face each other to the death in Zaroff's room. The fight, therefore, is fair, and Rainsford wins by killing Zaroff. As a result, yes, Rainsford is responsible for Zaroff's death. Zaroff even accepts the terms of this fight and freely participates when he says the following:
"I see. . . Splendid! One of us is to furnish a repast for the hounds. The other will sleep in this very excellent bed. On guard, Rainsford."
On the other hand, one could argue that Rainsford is not responsible for killing Zaroff because he virtually turns into a beast that simply fights for its life. Therefore, the argument could be that Rainsford is so far past rational human thinking at the end of the story that he reacts like a beast. In fact, Rainsford claims the following:
"I am still a beast at bay. . . Get ready, General Zaroff."
When Rainsford speaks metaphorically about being a beast at bay, he might also be speaking from a psychologically literal sense. For example, Zaroff forces Rainsford to act like a hunted beast for three days and nights in a jungle. Due to the mental trauma inflicted upon him over the course of these days, Rainsford starts to think and behave like a beast in order to survive. A "beast at bay" means that it has been cornered and must turn and fight to survive. Therefore, one could argue that Rainsford is not responsible for his actions when he kills Zaroff because he has been driven to the point of irrational thinking--thinking like a beast at bay and not as a human. One might also say that Rainsford could probably plead insanity for killing Zaroff and not be held responsible for his actions.