Since you mentioned both slavery and the Electoral College in the same post, it’s also worth mentioning that slavery is a big part of why we have the EC in the first place.
Remember the 3/5 rule? That was designed to give the southern states more representation in the House than they would have gotten if representation was calculated only by the number of free people.
This also had the effect of giving southern states more voting power for Presidential elections. If elections were determined by the popular vote, the south had very little power, as all the “3/5” people factored into their number of electors per state weren’t permitted to vote. Of course, this was all theoretical. Amazingly, the popular vote winner was also the EC winner in all antebellum elections (except for 1824, which was a 4-way mess that ultimately was decided by Congress).
We don’t have slaves anymore. We don’t have “3/5” people anymore. Every American citizen over the age of 18 (with the exception of people who have lost their voting right due to criminal activity) has the right to vote and be counted as a 100% person. Any system that gives one voter more weight than another is fundamentally unfair.