The Grapes of Wrath is a good example of mans inhumanity to man. One quote that John Steinbeck uses to describe this is when the Joads first arrive at the government camp. Ma says:
"We're Joads. We don't look up to nobody. Grampa's grampa, he fit in the Revolution. We was farm people till the debt. And then--them people. They done somepin to us. Ever' time they come seemed like they was a-whippin' me--all of us. An' in Needles, that police. He done somepin to me, made me feel mean. Made me feel ashamed. An' now I ain't ashamed. These folks is our folks--is our folks. An' that manager, he come an' set an' drank coffee, an' he says, 'Mrs. Joad' this, an' 'Mrs. Joad' that--an' 'How you getting' on, Mrs. Joad?'" She stopped and sighed. "Why, I feel like people again."
She is talking about the changes that have occurred to her at the hands of the government.
They random acts of anger that have received from total strangers has taken her by surprise and made her feel hostile herself. It makes you feel that she isn't "treating others the way you'd like to be treated" but she has evolved into "treating other the way she is being treated."
This brings up a good question that people often ask about the "good in the world." Constantly people are commenting about "what this country is coming to" in reference to all the cruelties going on. The reason for this is exactly how Ma has explained. They are struggling so much to stay alive not necessarily because of bad weather or bad land or bad luck, but because mankind has become unkind. She is realizing that the world is coming to a "save yourself" type and she needs to watch out...