Quote 1
The white children could tell their ages. I could not tell why I ought to be deprived of the same privilege. (1.1)
Sometimes it takes the mind of a child to see something so obvious that we've forgotten how to look. When Douglass is a child, he is confused about why he isn't allowed to know his own age, and it shows us something really basic about slavery: it doesn't make sense.
Quote 2
The fatal poison of irresponsible power was already in her hands, and soon commenced its infernal work. That cheerful eye, under the influence of slavery, soon became red with rage; that voice, made all of sweet accord, changed to one of harsh and horrid discord; and that angelic face gave place to that of a demon. (6.2)
As the saying goes: "Power corrupts, and absolute power corrupts absolutely." Douglass is talking about Mrs. Auld's transformation when she first starts to own a slave (him) and how that kind of "irresponsible power" corrupts her. She hadn't been a bad person up to this point, but having complete control over another human being transforms her from an angel into a demon.
Quote 3
Slavery proved as injurious to her as it did to me. When I went there, she was a pious, warm, and tender-hearted woman. There was no sorrow or suffering for which she had not a tear. She had bread for the hungry, clothes for the naked, and comfort for every mourner that came within her reach. Slavery soon proved its ability to divest her of these heavenly qualities. Under its influence, the tender heart became stone, and the lamblike disposition gave way to one of tiger-like fierceness. (7.2)
When Douglass is talking about how slavery changed his master's wife, Mrs. Auld, notice that he describes her transformation the same way he describes how slaves are "brutalized." She starts out a human being, with warm feelings and emotions, and becomes almost like an animal. Where she used to have sympathy for any person that was suffering, being a slave master hardened her heart until she became as cruel as a tiger.