The reason the world looks down on Americans for slavery is more how long it and then apartheid segregation was clung onto in an official, codified form. In the 1930s it was America inspired the Nazis, not even South Africa had a set of race laws so well developed at the time and Germany sent teams of lawyers to study how to replicate it for the reich. The US was very late and still struggles tear at society, that's the running sore.