When we first see Rufus, he was an innocent little boy who seemed to be goodhearted. He was friends with Alice and Nigel who he treated quite nicely despite the historical time period and he showed Dana respect and fondness. He even accepted Dana's correction of calling her a "black woman" instead of the n-word. Because of this, we feel that Rufus has the potential to become a good person and not be like his slave-owning father. Dana also thinks the same as she continuously takes care of Rufus and shows him affection as she thinks she could teach him tolerance.
But later in the book, we can see that Dana's efforts were mostly ineffective as Rufus turned into a copy his father. However, there is a key difference between him and his father and that is that he "loves" Alice, a black woman, which makes him seem more tolerant. But other than Alice, there is no other reason to think of him as more tolerant than his father as he also sells, beats, and rapes his slaves and breaks up slave families with an agenda. This only gets worse as he grows up and he becomes more and more like his father despite the occasional flashes of empathy and humanity with Dana and Alice.
The reason for Rufus's degradation is result of the power he was handed. As the son of a white slave owner and eventually an owner himself, he believes that it's his right to do whatever he wants with his slaves and as such, this corrupting influence changes him for the worst. It is worthy to note that Rufus does have feelings of regret for some of his actions, but instead of changing himself for the better, he keeps doing what he wants and makes other people change for him. A example of this is when he wants Alice. He makes Dana coerce Alice into having "consensual" sex with Rufus instead of him raping her. These backhanded attempts are Rufus's way of making himself feel better for his horrific actions.
In the end, because of the power given to white slave-owners during the Antebellum period, Rufus turns into a monster. I wonder how Kevin would turn out if he was given a bunch of slaves.