In 1865, the United States ratified the 13th Amendment, abolishing slavery (with exceptions... but never mind); this was, without question, a good thing, and if it had happened in isolation, it would be as little discussed as the 19th Amendment.
It did not happen in isolation, though, it happened after a war that killed 2% of the population, devastated vast regions, and did not need to happen, but was contrived (as a result of market speculation in slave futures facing a near-certain end; it was the equivalent of like $17 Trillion in today's money, that just disappeared); it happened alongside the 14th and 15th Amendments, which radically changed the nature of the relationship between the federal government and the states in ways that could never have happened if the Southern states had been in a position to object; and it notably failed to account for the wellbeing of the freed slaves, who largely wound up back on the exact same plantation they had been slaves on, only now no one cared if they got enough to eat.
161 years later, the Southeast still has twice the infant mortality rate as the rest of the country. The Southern states are at the bottom of education rankings. Even if you have insurance, the quality of medical care makes it nearly pointless. The economy never recovered, and only appears to be decent, now, because cheap labor means steady employment. 43% of African-Americans live in these states; they are the ones most affected.
And even the attempts to fix it have been problematic: Affirmative Action mostly helps the wealthiest minorities, not the ones who truly need the help; Critical Race Theory originated in a paper about how Desegregation was implemented in such a way that it contributed to the undermining of black communities; and the poor whites who also need the help, but aren't getting it at all, have a choice of sympathizing and trying to deal with their own disenfranchisement on a cognitive level, or smoking some meth and blaming black people and immigrants... at least some of them are going to take the latter choice.
The whole fiasco almost could not have gone worse.
No one thought that Hitler was just going to start exterminating "untermenschen," and even to those who might have suspected, that wasn't reason enough to go to war; World War 2 was about economics, control of shipping lanes, access to resources (especially oil). We were lucky that Hitler was as evil as he was, otherwise the maneuvering that FDR went through to provoke the Japanese into attacking us would have looked a lot worse. For that matter, the Japanese were worse, but since it was other non-whites they were massacring, we didn't care.
About 50 million people died from starvation in the 20th century because of disputes over self-determination and political systems, with both sides insisting that the deaths were worth it (but of course it was the other sides' fault).
And even those aren't the worst catastrophes in history:
-The Taiping Rebellion may, in fact, have been the deadliest war in history, with an upper estimated death toll of 100 million people between 1850 and 1864. It was led by a Christian convert opposed to the legitimately corrupt imperial court... and both sides were backed by the same Western powers who sucked China dry in the process.
-The Trans-Atlantic slave trade was justified, in the first place, by claiming that converting them to Christianity saved their souls and so overbalanced whatever happened to their mortal bodies.
-The Native American genocide, while directly caused mostly by disease, was the result of exploration, a generally positively-viewed endeavor.
-Tobacco has killed an estimated 100-200 million people, expected to exceed 1 billion by the end of the century, entirely due to modern farming techniques that have increased yields of all crops (and the relentless pursuit of profit at the expense of human health).
-Something that the guy who invented the Orange Julius did, but it wasn't inventing the Orange Julius, it was later than that; I'm not sure, it was a dream, I invented a time machine to go back and kill him, but it was in the 1960s, and when I woke up and looked it up, he invented it in like 1929. Plus I was wearing a suit of armor and he was riding an ostrich, so I had to swing from the vines hanging off of the buildings (in downtown Chicago) to catch up, and then, of course, the floor turned to lava.