Chapter 6 ends at the place where the story began: the banks of the Salinas River. In the first description of this landscape in the first chapter, the scene is full of animal life and the sun is shining. But, in Chapter 6, it is late afternoon and the landscape is in shade. A heron plucks a water snake out of the pool and eats it: a symbol of death (foreshadowing Lennie's death) and a symbol of the unfortunately necessary forces of nature. It is the latter symbolism that runs throughout the story. This is a story with naturalist elements. Naturalist works tend to exemplify characters who are subject to the conditions of their environments. Lennie and George are subject to the social realities of working on ranches. And they are also forced with the problem that Lennie does not fit into this social world. Therefore, the symbolic images (such as the heron eating the snake) represent this naturalist theme: that some are more equipped to survive.
With his voice, Lennie imagines Aunt Clara and a giant rabbit scolding Lennie for holding George back. When George arrives, he doesn't yell like he normally does. He agrees to tell Lennie how they are different from most workers because they look out for each other. And George reluctantly tells Lennie about the rabbits one last time as he prepares to shoot him.
"We gonna get a little place," George began. He reached in his side pocket and brought out Carlson's Luger: he snapped off the safety, and the hand and gun lay on the ground behind Lennie's back.
It is significant that George is using Carlson's Luger. This is the same gun Carlson used to shoot Candy's dog. The dog's death foreshadowed Lennie's death because the dog had become useless, not fit for this world, just as Lennie became too destructive for his own good. After Carlson shot the dog, Candy wished he had shot the dog rather than Carlson. In the end, George makes the decision to shoot Lennie before Curley or Carlson can get to him.