For Faulkner's "A Rose For Emily," O'Connor's "A Good Man is Hard to Find," and Erdrich's "The Red Convertible," one common theme is that of mental illness.
In O'Connor's short story, the Misfit is mentally unstable—he is the personification of evil. He seems to think little of killing Baily and his family. Killing comes naturally to him. He seems to be sociopathic (unable to feel guilt for his crimes) and delusional. He was put in jail for murdering his father, but he remembers his father's passing quite differently:
It was a head-doctor at the penitentiary said what I had done was kill my daddy but I know that for a lie. My daddy died in nineteen ought nineteen of the epidemic flu and I never had a thing to do with it. He was buried in the Mount Hopewell Baptist churchyard...
The Misfit has arranged details as he wants to believe them, and has repressed the actual event. The reader can easily believe he did kill his father. His comment, "No pleasure but meanness" seems to clearly expose the Misfit for what he truly is.
In Faulkner's story, Emily Grierson is controlled so completely by her father, that his death causes her mental break. For days after his passing, Emily will not allow them to take his body from the house. Her breakdown may be the result his treatment of Emily:
We remembered all the young men her father had driven away, and we knew that with nothing left, she would have to cling to that which had robbed her.
The loss of her father's interference is as distressing to her, perhaps, as his the exertion of his power over her was. The narrator also informs us Emily's aunt went insane.
Perhaps we should not be surprised that Emily has, in fact, gone crazy herself. By the story's end, we learn that the worst thing she did was not in the killing of her lover, Homer Baron, but that she was sleeping with his dead corpse—until the time of her death—many, many years after she poisoned him. We know this because Faulkner clearly describes the change of her hair over time. At the end, she has steel-grey hair. When they breakdown the bedroom door, they find the long-dead corpse. However...
Then we noticed that in the second pillow was the indentation of a head. One of us lifted something from it, and leaning forward...we saw a long strand of iron-grey hair.
The mental illness characterized in Erdrich's novel Love Medicine, in "The Red Convertible," affects Lyman's brother Henry, who returns from the war a changed man. He and Lyman owned the car together for years, creating many happy memories. Once they drove to Alaska. When Henry is away at war, the car is untouched. When he returns, he has no interest in it or anything else, suffering (it seems) from Post-Traumatic Stress Disorder (PTSD). Henry's fate is foreshadowed with...
We owned it together until his boots filled with water on a windy night and he bought out my share.
After the war, the brothers have a brief moment of clarity—as they had as youngsters, driving the convertible. At the lake, they fight, then they laugh. Henry expresses his complete hopelessness. Lyman tells him to "wake up;" Henry says...
I know it...I know it. I can't help it. It's no use.
Henry's depression seems too great to bear; he quietly walks into the water, going farther and farther out until his head goes under. Lyman tries to find him, but the water is moving too quickly. So he turns the car and its lights on, and pushes it into the water, to join his brother.