Douglass illustrates that it doesn't matter if slaveowners are good or bad people, because the institution of slavery corrupts all the owners. They simply have too much power and the slaves too little. He also says that the owners not unreasonably fear the slaves will rebel against such an unfair situation. The masters believe they must use fear and cruelty to keep the slaves abject.

He talks about his horror as a young boy at seeing a female slave stripped to the waist and beaten until blood ran down her back by her master for disobeying him. In another instance, he answers charges that slaves, when asked, will say they are happy with their lot. He tells the story of a slave who had never seen his master because the plantation was so large and so many slaves worked the fields. When he was approached by a white man and asked if he were happy, he told the truth about the misery of his condition. The slave later found out that the man was his master. This slave was very openly used as an example to the other slaves not to complain and sold south to a cotton plantation where the likelihood was that he would be worked to death. This kind of behavior on part of the masters frightened and silenced the slaves.

Douglass is very clear in wanting to explain that slavery isn't an issue of good or bad individual slaves owners. He rejects the argument that if slave owners could simply become kinder, the whole situation would be fine. His point is that the institution is an evil that corrupts everyone involved in perpetrating it. While he does show that social conditions can make the lot of the slave easier—he notes that slaves in the city of Baltimore are treated better than slaves on isolated plantations because neighbors can see and hear what is going on, so masters curb their brutality—the institution itself is the problem. It can't be "tweaked"; it must be abolished.

Douglass speaks about the hypocrisy of slaveholders and claims that the most religious ones are often the most severe. Religion does not save their souls but makes them feel entitled to inflict cruelty. He says that following his owner's conversion to religion, his owner became even more brutal and inhumane:

Prior to his conversion, he relied upon his own depravity to shield and sustain him in his savage barbarity; but after his conversion, he found religious sanction and support for his slaveholding cruelty.

In other words, Douglass writes that his owner uses religion to justify his barbarity. Wearing the cloak of (false) religion makes his owner feel more justified to perpetuate even greater cruelties, because he excuses them in the name of religion. Slave-owners also break up the religious school that Douglass organizes with some other slaves, and the owners use religious holidays as times to encourage their slaves to become dissipated. The owners prefer their slaves be drunk so that the slaves won't be able to muster the strength to rebel. Douglass believes that owning slaves is so morally corrosive that it is completely incompatible with being a true Christian. 

I think that one of Douglass' strongest points is that he depicts slaveholders as being afflicted by the institution of slavery.  This condition causes them to be sadistic and unspeakably cruel.  In Douglass' writing and thought, slaveholders are cast in this light.  Sophia Auld is an example of someone who became morally corrupt due to slavery.  Douglass points out how Auld had been kind and decent to him precisely because she had never owned slaves before.  Yet, as she becomes a victim to the institution of slavery, her moral bankruptcy accelerates and she becomes as cruel as any slaveowner:  'The fatal poison of irresponsible power was already in her hands, and soon commenced its infernal work.’’  Such a depiction reveals how slavery can dehumanize both the slave and the person who owns slaves.

Yet, this is not to say that Douglass shows slaveowners to be victims who have lost their souls.  Edward Covey is shown to be an individual of the slave owning institution who knows very well what he is doing.  He is clearly abusive and clearly a perpetrator of evil.  He sneaks up on slaves, earning the nickname "snake."  Covey is cruel in the way he treats and "breaks" slaves.  While Auld might be seen as a victim of the institution of slavery, Douglass still regards her as part of the problem.  Covey is the embodiment of the problem.  In this light, Douglass is able to show that there can be different variations on the same theme of cruelty that exists within the slaveowning community.