To me the answer depends on why was slavery not abolished?
If slavery remains in practice because enlightenment failed in America and economic and racist interests won, then (from the perspective of a reader today) anyone supporting slavery must be "bad", and your protagonist can only be good if he is secretly against slavery and working towards abolishing it or at least attempting to relieve his slaves from as much of their suffering and granting them as many freedoms as possible. But then I don't see the point in your alternate history, because some slave owners in history acted that way, and all you would have done is set it at a later date.
If on the other hand your society is one where slavery is not racist and commercial, then any person can be good or bad, just as in a society without slavery. If you look at history, there are societes where slavery wasn't a result of slave trade, but of war. For example in ancient Rome, slaves where prisoners of war who were made use of, instead of being put to death or left to rot in jail. They were brought into Roman society as slaves as a way to civilize them and turn them into Romans. Their slavery was not exploitation so much as reeducation. And consequently it wasn't rare for Roman slaves to be set free eventually and even rise in society. Sure, you had the gladiators and other slaves that were used as cheap labor, but then many Roman citizens lived in much worse squalor and slaves were not treated worse than a poor Roman farmer. Because in Rome, there was no racism at the root of slavery. Slaves weren't considered animals, but people.
So if you want a "good" slave owner, then you need a society in which slavery is not based on commerce and racism. It could be a world where the United States have become an empire like the Roman empire, expanding by conquering new lands, and many African countries have been taken into that empire. Slaves then are prisoners of war from newly conquered lands. If they are educated, they are teachers of the children of the slave owners; if they are uneducated, they work in the kitchens and fields. They are treated well, sent to school (because the goal is not exploitation but education and civilization). And if they have led good American lives and acquired American values, they are eventually rewared by being set free and considered American citizens like all others.
If slavery is not racist but educational, and if "good" slaves are rewared by being set free and accepted as citizens, then a slave owner can be a good person, because his role is that of a supportive parent and teacher.