In Letter 38, Sofia describes the ways in which she altered her behavior in order to survive imprisonment. Her statement contains multiple illustrative oxymorons:
I’m a good prisoner, she say. Best convict they ever see. They can’t believe I’m the one sass the mayor’s wife, knock the mayor down. She laugh. It sound like something from a song. The part where everybody done gone home but you. . . . Good behavior ain’t good enough for them, say Sofia. Nothing less than sliding on your belly with your tongue on they boots can even git they attention. I dream of murder, she say, I dream of murder sleep or wake.
Sofia describes herself as a "good prisoner" and the "best convict they ever see." Most people implicitly associate convicts with violence and bad behavior, rather than with the qualifiers "good" and "best." These oxymorons undermine the culturally-ingrained assumption that all convicts are perpetrators of violence, rather than victims themselves. Sofia is forced into defending herself and then villainized for it—arguably, she is more victim than criminal.
Walker uses oxymoron in this passage to highlight the racist and hypocritical expectations of white people in power—namely, the expectation that people of color who defend themselves from violence through the use of violence are in the wrong. Sofia is a "good" prisoner and a "good" Black person in the eyes of the white supremacist state because she does not fight back.