Why does Slim understand about what George had to do that neither Curley nor Carlson understood?

Slim understands Lennie's fate after he (Lennie) kills Curley's wife. He knows that Lennie could hardly understand how and why he killed her. Still Curley is wise enough to know that there will be no trial or mercy for Lennie. Mental challenges such as Lennie's were not understood or really even tolerated during that time,

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." (pg. 96)