And there were people against slavery in America at that time as well so it wasn't impossible to come to the right decision.
People are against abortion today. Would you say that our great-great-grandchildren will be right to assume that every pro-choicer was a hate-filled, mass-murdering, human rights atrocity? Had
they lived now, no doubt, they would have founded active resistance movements to save children from murderous so-called "doctors" with their hands soaked in the blood of the innocent; they would have
boldly stood for right, and...
...but of course, most wouldn't. Most folks are people, and our world defines much of how we look at it.
Hell, people eat meat today. A century and a half from now, they might look at our meat industry as inhumane, and ask how their ancestors (us) were so cruel, so idiotic, so inhuman, as to eat other living creatures.
Slavery as it was practiced since humankind first realized it was easier to force their neighbor to labor on their behalf than do the labor themelves is evil. Chattel slavery was forbidden in the New Testament (bondhood was modified - eventually as the West Christianized the practice partly died out, and partly was subsumed by feudalism), and those who wanted to build a case it was evil in the late 18th Century certainly had a lexicon to build on. In fact, they did so, which led to the 19th Century.
But ending one evil does not necessarily make for wise policy if the result is to replace it with a greater evil. We survived the Civil War not least because we had a century and some change under our belt operating as a nation - our institutions were developed, and capable of withstanding the blood. Even then it was a close-run thing. A century prior?
While attempting in all things to align our will most closely to that which is Good, we should retain some basic humility when it comes to the probability that future generations will look at us and identify at least as many (if not more) flaws than we identify when we look at our own forebears, and judge those forebears accordingly.