The most important thing to keep in mind is that the US was not "a country" but 13 different countries with competing interests operating under a "Confederation" agreement. After a few years, it became clear, that the Confederation agreement had problems especially with respect to "trade, foreign policy, and defense".
These independent countries had to build a system that prevented a subset of the nation to dominate the government.
From the start, small states like Vermont feared the larger states like New York and Virginia, so to get the small states on board, the idea of a bicameral Congress was agreed upon, where the House had the most power (since it controlled spending), but the Senate had the power to block most laws that might impose the will of larger states on smaller states through their disproportionate representation in the House). A similar concept in the electoral college watered down the power of the big states.
Before attributing the electoral college to chattel slavery, it is important to look at how the EU addressed the same issue.
The EU uses the concept of degressive proportionality.
"The allocation of seats to each member state in the European Union is based on the principle of degressive proportionality, which means that while the size of the population of each country is taken into account, smaller states elect more MEPs than is proportional to their populations"
Sounds familiar, doesn't it.
The EU does not have slavery and so there is no provision to account for slaves in its allocation of seats in its elective body or anywhere else in its governance. But the concept of disproportionate representation for smaller states exists in the EU just like the US.
The southern slaves, knowing how strong the abolitionist movement was in the north sought to neutralize the ability of northern states to end slavery, this was accomplished more in the House and Senate then in the electoral college.
However to balance the power of the northern states, both the north and south agreed to the 3/5th Compromise that inflated the representation of the South in the electoral college and the House. Without this compromise slavery may have continued in the South for several decades longer.
By agreeing to the compromise and the over representation of the southern states in the House and electoral college, the northern states in fact put in motion the forces that less than 100 years later would end slavery.
The forefathers were a mixed bag, some for slavery but most against. Both sides realized this inherent conflict would not stand forever. But the issue of the electoral college had little to do with slavery, it was the 3/5th provision that was a direct compromise made to allow the country to come together.
