In considering the ending of Steinbeck's Of Mice and Men, it is important to remember that the novella has been written in the style of Social Realism. While this work is a sympathetic portrayal of the lives of the bindle-stiffs during the Depression, it also exposes the injustices and inequalities of life and the struggle of the strongest to survive amid loneliness and alienation.
With the objective narration as mere observation, there is the distance of the Realistic, as well. Like an animal, Lennie falls victim to his own nature--it is nothing that he can control--and life goes on. As he hides, Lennie imagines a large rabbit talking to him. Symbolic of safety, the rabbit chides him,
"You ain't worth a greased jack-pin to ram you...George done ever'thing he could to jack you outa the sewer, but it don't do no good...."
The mentally challenged Lennie represents, by Steinbeck's own words, "the inarticulate and powerful yearning of all men," the frustration of the itinerant workers during the Great Depression who yearn for identity and stability. In the socially realistic style, Lennie does not survive because he has the inability to adjust to his environment on his own.