Why did Bob lie about what happened when he gets back to camp in the end?

Bobbie lies when he gets back to camp because he doesn't want to admit that he killed David. Although David, paralyzed from falling fifty feet over a ledge, asked Bobbie to put him out of his misery by pushing him to his death, Bobbie would almost certainly be indicted for murder were he to confess what he'd done. And so he doesn't let on. Instead, when he staggers back to camp after his terrible ordeal, he tells everyone that David fell straight down to the ice, just where his body was found.
David has lost his life, and Bobbie has lost his innocence. As he tells us, this was the last day of his youth, which was left up there on the mountain, along with the dead body of his late friend.
https://rpo.library.utoronto.ca/poems/david