The character this question refers to is Roger. In Chapter Eleven, Roger uses the lever to let loose with a boulder that could kill Jack, as easily as it could have killed Ralph or anyone else. After heaving the boulder, Roger finds himself in trouble with Jack once more, as he is chastised for leaiving his post.
"High overhead, Roger, with a sense of delirious abandonment, leaned all his weight on the lever."
"Presently the tribe returned noisily to the neck where Roger joined them.
The chief spoke to him angrily.
"Why aren't you on watch?"
Roger looked at him gravely.
"I just came down--"
The hangman's horror clung round him."