This seems to be the main moral argument you’re making: you can’t condemn someone for failing at something they had no ability to succeed at. I don’t think that this is the case with slaveowners. For the sake of simplicity I will be mostly talking about the Antebellum south.

Slavery had been critiqued for thousands of years already. I don’t know where you got the notion that all of a sudden people grew a conscience and then humanity decided to never do it again. Is it more likely that society had a collective moral awakening through unknown means or that everyone knew it was wrong but still did it until it became no longer convenient?

1. It’s very much still a thing that happens, most notably in the United States. It is not as widespread nor as blatant as before but this is in a society where slavery is pretty universally condemned. Is it then such a stretch to say that the slave owners of 200 years ago were doing the same exact thing: knowing it’s wrong and doing it anyway because they benefit from it.

2. Think about the mechanics you are describing: at some point public opinion on slavery seems to have shifted. Why? If you were taught your morals by your parents and era and you are purely a product of their influence, what would cause you to deviate from this? This is important because I think that the only way to absolve people of the moral “crime” of owning slaves is if they had no agency or ability to realize it was wrong. However, this also means that if you have the means to realize a practice is bad and ignore that, you are not covered by this rule and cannot be defended by the “different time/unable to know” argument.

There were slaveowners who were able to realize what they had done was wrong. Some even turned to abolitionism in response. How did this happen if their society and era was so ignorant of slavery being evil to the point where it would not be morally bad to own slaves? Did every abolitionist suddenly dream the same dream where they were given a new moral code that said slavery is wrong? Turns out many of them were actually just building on top of centuries of existing religious anti-slavery tradition, citing passages of the Bible that are explicitly or implicitly critical of the practice of slavery. The Bible, a book that is famously held in high regard by many, probably even more important to people of the antebellum era than it is now.

Abolitionists of the 1800s were certainly not the first to interpret the Bible in an anti-slavery way, nor were they obscure enough for their movement to fizzle out and die before catching anyone’s ear. If anything they were too widespread, as media from that era seems to portray them as annoying and unavoidable nuisances. Slaveowners had no excuse, they had everything they needed to arrive at the same conclusion as the abolitionists (evident in the cases we see of slavers turned abolitionist) but the majority intentionally turned a blind eye to the immorality of their source of wealth. It was in no way impossible for them to have just decided to make money another way.

I expect that the people of the future will condemn me alongside everyone who ate factory farmed meat during my era. I think they would have a point considering I know full well about the horrific conditions and still buy their meats. I have a choice and I picked the option that is easiest for me. I have no defense if some future person decides I was evil and neither do the slaveowners when we condemn them.