In the US at least, these two events are directly or indirectly linked to two of the worst wars this country has even been involved in.
The Holocaust, having been so unimaginable in what was considered a highly sophisticated society, and having occurred relatively recently and right in the middle of the latest World War, is impossible to ignore. I mean, we are talking some of the greatest minds in world history--artists, scientists, etc--getting slaughtered. Millions--men, women, children. Based on being Jewish.
The **** Socialist Party. (National Socialist German Worker's Party)
Giant centralized government at it's worst...unless we also look at the 10s of millions murder by China's Socialist party...or Russia's, or....well, you get the picture.
Nuts
Slavery has happened in almost every society that ever existed--and still occurs in places in Africa. But in a nation claiming to be home of the free, well, even if it was 150-200 years ago, it stood out more than others.
And then, of course, we had the American Civil War as a partial result of it.
Also nuts
The USA still carries the monetary and social baggage of slavery in 2016. Billions ( trillions?) of dollars in government programs over many decades, ongoing racial division which actually got worse the last few years, etc, etc.
The price was and is MASSIVELY steep indeed. The US will continue to pay for this.