Slim turned quietly to George. "I guess Lennie done it, all right," he said. "Her neck's bust. Lennie coulda did that." George didn't answer, but he nodded slowly. His hat was so far down on his forehead that his eyes were covered. Slim went on, "Maybe like that time in Weed you was tellin' about." Again George nodded. Slim sighed. "Well, I guess we got to get him. Where you think he might of went?"
It seemed to take George some time to free his words. "He- would of went south," he said. "We come from north so he would of went south."
After the men discover Curley's wife's body and decide it was Lennie who killed her, Slim pulls George aside and there is a change in tone. What is the significance of this change, as demonstrated in the above excerpt?