When Rainsford calls himself a beast at the end of the story, what does the description foreshadow in "The Most Dangerous Game" by Richard Connell?

When Rainsford calls himself a beast, it foreshadows him killing Zaroff. 

Foreshadowing is when the author hints at something that will happen later in the story.  When Rainsford first met General Zaroff, he considered his game of hunting human beings murder.  He could not believe Zaroff was for real at first, and then told him that there was no way he would participate as a hunter.  He had no idea he was going to be the game. 

Zaroff tells Rainsford that his idea about hunting people being murder is old-fashioned.

 “…I'll wager you'll forget your notions when you go hunting with me. You've a genuine new thrill in store for you, Mr. Rainsford."

"Thank you, I'm a hunter, not a murderer."

"Dear me," said the general, quite unruffled, "again that unpleasant word. But I think I can show you that your scruples are quite ill founded."

Rainsford has a difficult time accepting his role as prey.  He gets somewhat intimidated and discouraged when he learns that Zaroff is just playing with him, so he can extend the game. Rainsford gives it everything he has with evasion and traps, and makes no headway.  

He decides to change the rules of the game.  Zaroff has a large chateau, and Rainsford figures out how to swim to it.  Zaroff is excited and stunned.  From his point of view, Rainsford has been a worthy opponent, and now he has won the game.   

Rainsford has other ideas. 

Rainsford did not smile. "I am still a beast at bay," he said, in a low, hoarse voice. "Get ready, General Zaroff."

The general made one of his deepest bows. "I see," he said. "Splendid! One of us is to furnish a repast for the hounds. The other will sleep in this very excellent bed. On guard, Rainsford." . . .

When Rainsford says he is a beast at bay, he is telling Zaroff that the game is not over.  Zaroff knows this, and is willing to keep fighting.  He loves a challenge. He just was not expecting it given Rainsford's views on murder.   Rainsford wins, and kills Zaroff.  Zaroff’s death was foreshadowed, or hinted at, by Rainsford saying he was still a beast.