Everyone is the book is also an animal, some vicious. Men kill animals out of spite or by accident; men fight each other out of spite; men kill women by accident; men kill men out of spite: it's a feeding frenzy.
The novella is very anthropomorphic and full of social Darwinism. According to Steinbeck's belief in Pragmatism, man is a product of evolution, an animal. Evil comes from ignorance and failure to adapt to the environment:
This ability to be flexible is another aspect of pragmatism, an ability that Steinbeck feels is fundamental to the survival of the migrant workers. Pragmatism also includes a movement away from abstract religious beliefs, concentrating instead on the holiness of those who are living.
So, in the 1930s, during the Great Depression, when food, natural resources, jobs, and homes were drying up, men began fighting for survival. Also, the top of the capitalist "food chain", corporations, began swallowing up all those at the bottom, causing the bottom feeders to feed off each other, instead of uniting to fight against the real power brokers.
Steinbeck is also a Populist: he fights for the little guy, the underclass, the injured, homeless, poor, farmers. He saw his idyllic Salinas River Valley taken over by corporations, and so he writes mixes social protest with biological forces to show the common man's will to survive.
Everyone in America was damaged at least in some small way by the Great Depression - even if not personally, then people knew someone else in the family or group of friends that had been hurt in some way. In the novel Of Mice and Men the author John Steinbeck tries to represent this. For example, lots of hired hands lost their jobs and had to become nomadic as they searched for work. Even millionaires did not escape - in fact some of them had it the worst. People were damaged mentally and some even took their own lives, jumped from skyscapers when they realised their thousands of dollars of assets had gone. So society was scarred from the top to the very bottom. Ironically, only those living "on the fatta the land" could eat well and subsist.
I think he does this to prove that all people have weakness. It is easy to judge Lennie as a big dumb oaf in this book. However, all people have something that makes them less than others in one way.
Steinbeck sees people. The Great Depression was an era that he wrote about that exposed people at probably their worst and their best. When push comes to shove, some people rise to the occasion. For others, pressure causes them to crack at the one point of their life's disability.
I think being a shorter novel, he took advantage of the way he developed character so that this feature of everyone's weaknesses would remind us as readers to look at our own and the impact it has on others. This will give readers opportunity think about how they can contribute their strengths to others weaknesses for the better.
In the 1930s John Steinbeck met a marine biologist named Edward Ricketts, whose views on the interdependence of all life greatly influenced the author's thinking. Steinbeck intermingled Ricketts's ideas with those of the psychologist Carl Jung, such as "the collective man." This concept of "the collective man" is prevalent in Steinbeck's great work as well as his highly regarded novella, Of Mice and Men.
Perhaps, then, in Of Mice and Men, Steinbeck portrays the men as inadequate as they are separate from one another. For, it is out his fear that someone may take his wife that Curley is belligerent toward the other men. It is out of fear that Crooks yells at Lennie and Candy enter the barn, it is out of fear that someone will discard him as his dog has been discarded that Candy berates Curley's wife. In their aloneness, these men become fearful and insecure. The naturalistic world of the Great Depression displaces the men as they search for work. It is an antagonism and fear that enters the lonely desperate hearts of the men, the common men who are exploited by the wealthy. This concern for the common man is prevalent in many of Steinbeck's socialistic novels.
In my opinion, this is because of Steinbeck's overall world view at the time when he wrote the book. To me, one of the major themes of the book is that no one really has it good in life (or at least not in the group of people he is looking at). All of these people are the ones whose best laid plans go awry.
So I think he makes everybody damaged, as you say, to show that the world is a tough place. He is showing that everybody has things really bad.