The mistress, Mrs. Auld outdid her husband when her she changed from being kind to Frederick Douglass to being harsh and cruel. She also stopped educating him.
The Mistress in question is Mrs. Auld for whom the slave Frederick Douglass worked.
Initially, she viewed all as equal and treated Douglass with respect until she was told by her husband to stop teaching Douglass how to read and treating him with kindness.
Suddenly, her kindness towards Frederick changed to cruelty to the point of outdoing her husband.
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