“Couldn’ we maybe bring him in an’ they’ll lock him up? He’s nuts, Slim. He never done this to be mean.”
Slim nodded. “We might,” he said. “If we could keep Curley in, we might, But Curley’s gonna want to shoot ‘im. Curley’s still mad about his hand. An’ s’pose they lock him up an’ strap him down and put him in a cage. That ain’t no good, George.”

– John Steinbeck

Of Mice and Men, Chapter 5. George is wondering what he should do about Lennie after Curley’s wife’s body is discovered. But Slim understands Lennie’s fate and that there will be no trial or mercy or for Lennie. The kind of mental problems Lennie had were not understood at that time. Slim appears to be encouraging George to carry out a mercy killing of Lennie.