Therefore, I think that the dog is a symbol of the treatment meted out to the old or those considered inconvenient or no longer of any use. I believe, this is an obvious metaphor for what George must do to Lennie, who proves to be no good to George and no good to himself. Steinbeck re-emphasizes the significance of Candy's dog when Candy says to George that he wishes someone would shoot him when he's no longer any good. And when Carlson ends up shooting the dog, Lennie is the only man not inside the bunk house, I think Steinbeck placed him outside with the dog, away from the other men, to somehow put him in or show the position he will be in at the end of the story. He also makes use of the same gun at Lennie's end; the only difference is that George is the one holding the gun.
I also think the implication of the omens in of mice and men are that the death of Candy's dog is a direct parallel, by this I mean that it almost describes in detail what happens to Lennie at the end. For example, the dog is shot in the back of the head; Lennie too is shot in the back of the head. I found out from reading "of mice and men" that some of the primary characters lose a loved one or someone they care about, e.g. in of mice of men Candy has to sacrifice his beloved dog for the good of others, and at the end George has to sacrifice Lennie for the good of others even though he loved Lennie.
I do not think that the treatment shown to the old, handicapped or those incapacitated in "of mice and men" is right or proper. I feel this way because I believe that everyone, irrespective of how old they are or