The eventual cruelty of Douglass's mistress, Mrs. Auld, had a greater effect on him than her initial kindness. At first, Mrs. Auld, who had never had a slave before, was "a woman of the kindest heart and finest feelings" (page 19). She did not realize that it was considered dangerous to teach a slave to read, thereby encouraging the slave's independence of thought, so she started to teach Frederick Douglass to read when he was young. Upon discovering what his wife was doing, Mr. Auld told her "that is unlawful, as well as unsafe, to teach a slave to read" (page 20). Mr. Auld feared that teaching a slave to read might make the slave rebellious and more prone to escape. Frederick Douglass said at this moment that he understood how the white man kept the black person enslaved, and "I understood the pathway from slavery to freedom." (page 20). He realized that if reading was so dangerous for slaves, the key to freedom was learning how to read. Frederick Douglass's acquisition of the ability to read allowed him to have the psychological power and tactical means to escape northward. Therefore, his mistress' cruelty had a greater effect on him than her kindness. 

Arguably, the cause and effect of Mrs. Auld's initial kindness and eventual cruelty has the greatest effect on Douglass.  When Douglass first meets Mrs. Auld, who never owned a slave, he is surprised by how well she treats him.  When she begins to teach him to read and write, he welcomes the opportunity.  Afterwards, when Mrs. Auld turns and treats Douglass with the cruelty that he has known from other white people, Douglass is struck, not necessarily by the cruelty alone, but by the fact that one who was so kind could be so quickly and thoroughly engulfed by racist ideology.  Douglass reckons that if someone as good as Mrs. Auld was at first can fall into the terrible grips of racism, then anyone can.  So, it's the cause and effect, or combination of Mrs. Auld's two personas, that has the greatest effect on Douglass.