This is a great question. There are many instances where the world is portrayed as brutal and indifferent. In light of this, let me give three examples.
First, when Candy's dog was too old, the men in the bunker wanted to kill it. They said the dog was too old and too smelly. Candy said that this dog was the best and served well all his life. In the end, the men's opinion prevailed and the dog was shot. The deeper meaning is that when an animal or person has passed the age of "service" it should be put out of its or his misery. Life is cheap and utilitarian. Candy is now an old man. He is also expendable.
Second, the world is also racist. Crooks is the sole black man on the farm. No one talks with him, and he has no community. He is alone socially and spatially. He just exists.
Finally, when it comes to Lennie, it is clear that no one really cares about him, except George. There is no compassion for those with mental disabilities. No one seeks to understand him or give him a chance. All of these points show that the world is a brutal place.