During the Great Depression, American people suffered from unemployment, crashing stock markets, and unaffordable daily products. In addition to these problems, people were taken advantage of by corrupt police and unfair business owners. People living through these times learned to judge what is right and what is wrong on their own, rather than relying on others' judgments. Steinbeck expresses the idea that when justice fails people take matters into their own hands through the plot of his books, Of Mice and Men and The Grapes of Wrath.
The most obvious way Steinbeck shows this is through external conflicts. One example of a conflict in the novel The Grapes of Wrath is when a young man hired by the large business owning the farms comes to destroy the home of a farming family with a bulldozer. The father threatens the driver with a rifle. His desperate attempt to persuade the man out of crushing his house has failed, and sensing the injustice, he decided to take action based on his own standpoints.
The action he took was not necessarily justified, but it satisfied his personal needs. Tom displays this kind of behavior as well, against the corrupt policemen. Two times he beats a policeman, killing an officer the second time. Both times his aggressive acts were caused by unjustifiable violence done by the cops against innocent people. The first time was when an officer shot an old lady who was completely innocent, and the second was when casey was killed by the policemen. He knew the police could not be relied on, so he took the matter into his own hands and decided to attack the cops.
The same thing happens in Of Mice and Men. When Lennie kills Curley's wife, the judicial system is not enough for Curley.