What training does Sophia Auld receive in "irresponsible power," changing her naturally kind disposition?

Douglass watched her transformation with a heavy heart. When he first came to her, she did not see him as chattel. She treated him like a human being and took care of his basic needs. Her cessation of instructing Douglass was her first step on the road to ruination. She went above and beyond her husband's request to leave off teaching her slave letters, and soon was most vigilant in making sure Douglass was nowhere near a newspaper.