If you are going to pass judgement on 19th century Americans by using 21st century mores and values then you are going to get a very distorted view of history and come up with very unreasonable conclusions..
So far I see two standards for both time periods that were guiding people.
1) Bible guidelines of what is right and wrong.
2) “Other” which includes economics, power, politics which are closely related.
There are these “touchstones” that we test what is true and right.
Slavery has existed from the beginning of time and will be there until the end. It is in the Bible and in some ways one can perceive it has condoning but the bigger picture is that all men are equal in opportunity and before the law.
The power/economics/political standards were on both sides of the fences.
So have the mores and values really changed?
The model for slavery in the 19th century wasn't master - tortured slave. That is ridiculous, the model would be parent - child. But the child would never be capable of ever taking care of him/herself on their own. No amount of education could raise the slave above his primitive natural state if left to his own devices.
Freeing the slaves was thought a cruel thing to do to a childish/childlike person, only abolitionist were that "evil" to think that freeing slaves all at once with no consideration of the slaves ability to cope was a just thing to do.